In China, most cities have their own “city flower”-a famous local flower representing the city.

My city-Fuyang, in Anhui Province, also has its flower-the Chinese rose, which is the best flower in my area.

I'm not really sure what you mean by "which is the best flower in my area," what about it is the best? The following are some of my guesses for what you might have meant.
"which is the prettiest flower in my area"
"which I think is the best flower" (This means that you would think it is better than all other "city flowers." In this case it's okay to leave the meaning of "best" vague, because it is an opinion, rather than a fact.)

These days, I suddenly noticed that the flower is blooming and at its best.

It doesn't make sense to use "suddenly" when you're talking about an interval of time (These days), because "suddenly" is something that happens in an instant.

I don’t even know when the Chinese roses started to bloom; it seems like they've bloomed overnight.

or "they have". You need this because of the pasted-tense of "bloomed."

The first Chinese character means month.

ahh! What does the second character mean! (You don't actually have to respond, since I can look it up, but it just seems weird to explain the meaning of only half of the word, especially since I think it helps with the explanation in your next sentence.)

It’s said that this flower can bloom every month, but I don’t know if this is true as I've never tried to proveested it.

Your sentence was correct, it's just that the phrase "I've never tested it" is more common than the phrase "I never tried to prove it."

The colors of the Chinese rose are various, whichst in spectrum and can be white, red, pink, yellow, orange, purple, etc.

Alternate suggestion that sounds a little more "natural":
"There are various colors of the Chinse rose: white, red, pink, yellow, orange, purple, etc."

Most of the flowers are planted along the streets, which are designed this wayas such: a plant belt in the center of the street, and two plant belts on both sides of the motor vehicle lane are two plant belts,; separating the motor vehicle lanes and non-motor vehicle lanes.

Since you start your description with "a plant belt," it makes more sense to follow up your list by starting off with "and <something about plant belts>."
Also, despite a semi-colon being "Proper English," I much prefer to have my asides in parathesis, e.g. "and two plant belts on both sides of the motor vehicle lane (separating the motor vehicle lanes and non-motor vehicle lanes)." And I think this is much more common amongst most Americans (I can't speak for other native English speakers).

PThe pedestrian lanes are on the outer most part of the street where there are also two plant belts.

I think "outmost" is still correct (I've at least heard it used by native English speakers), but "outer most," is "more" correct.

Imagine the flowers on the five plant belts blooming.

Since you just used "5 plant belts" in the previous sentence, it doesn't need to be used here (it sounds redundant). But if you did still want to use "five plant belts" you could restructure your sentence to the following.
"Just imagine the flowers on all five plant belts blooming!"

What a great sight the they are!

Correct, other suggestions:
"What a beautiful sight the they are!"
"What a wonderful sight the they are!"
"What an amazing sight the they are!"
"What a pleasant sight the they are!" (This one is slightly less "powerful" than the others)

In China, most cities have theira “city flower”- - a famous local flower that representings the city.

It's a little odd to use a possessive when saying THAT something exists in this way. Instead, we'd use "a" or "an". For instance, "Most of these cars have a special type of engine" or "This house has a mysterious, abandoned room". But then we might use the possessive when the possessed object is a subject: "His car runs on diesel fuel".

Also, "representing" works perfectly fine, but for some reason, it feels slightly better to me as "that represents" or "which represents" in this context.

Also, when using a hyphen to separate clauses instead of as a way to contract words together, we'd use spaces. Technically, it should be an em dash ( – ), which is slightly longer than a hyphen ( - ), but this character isn't on most English keyboards so it is only used in very serious writing.

My city-, Fuyang, in Anhui Province, also has itsa flower- - the Chinese rose, which is the best flower in my area.

These days, I'm suddenly noticeing that the flower is blooming and is at its best.

Alternatively: "...the flower is blooming, and at its best [too]."

I don’t even know when the Chinese roses started to bloom; it seemsed like they bloomed overnight.

It’s said that this flower can bloom every month, but I don’t know if this is true as I've never tried to prove it.

"I never" feels more... contemplative?.. to me than "I've never", which feels more casual and matter-of-fact. "I never" feels like you are contemplating a long period of time, perhaps your whole life, because something very significant has happened. It carries with it (for me, at least) a sense that what you didn't do is something you'll NEVER get to do in the future, because something has come to an end. Like if an online game has just shut down permanently, someone might say "No! I never got to explore the final area of the game!"

Meanwhile, "I've never" or "I haven't" carry a sense that it is something you haven't done yet, but COULD in the future. For example, if that online game hasn't shut down yet, a player might casually say: "Oh, I haven't been to the final area." Whether they plan on going there or not, someday they potentially could, so it feels to me like it should be "haven't".

The colors of the Chinese rose are various, whichand can be white, red, pink, yellow, orange, purple, etc.

Most of the flowers are planted along the streets, which are designed this way:like this: there's a plant belt in the center of the street, and on both sides of the motor vehicle lane are two more plant belts, separating the motor vehicle lanes and non-motor vehicle lanes.

"like this" feels like it wants to point forwards in the sentence just a little more than "this way". "This way" feels like it's referring to something that has been said BEFORE in the sentence, rather than something that will be said AFTER. Unless the sentence is very short.

Most of the time, anything after a colon could also be its own sentence. So I've added a "there's" at the beginning to fit with this and make it sound more natural.

"motor vehicle lanes and non-motor vehicle lanes" could be contracted down to "motor and non-motor vehicle lanes".

Also, it feels a little better to me to say "more" before the extra plant belts, to emphasize that they are new ones and not ones we've talked about before.

Pedestrian lanes are on the outmost part of the street where there are also two more plant belts.

I often say "outermost" instead of "outmost" for some reason. But both are fine.

Imagine the flowers on theach of those five plant belts blooming.

This feels a little more natural to me in this context, though it's very hard for me to explain why. Alternatively, if we removed the "five plant belts", it might be: "Imagine those flowers blooming." But yet again, we're using "those" (and "each of" will be used for extra emphasis if it is a specific quantity of things). Otherwise it feels weird to me, because this is something that has more emotional impact, not something mundane or kind of silly.

Some examples: "Imagine the looks on their faces when they've found out what we've done!" or "Man, imagine the chaos that would ensue if they ran out of supply", versus something like "Just imagine the faces of those poor children who have been affected by the crisis" or "Imagine that beautiful, golden sunset over the town".

What a great sight the they are!

I would probably gravitate towards saying something like, "They're so beautiful!" or "They're such a great sight!", whenever I'm talking about something in this way. Especially about objects that I like.

I amt makes me a little sad that I can’t post photos and videos about our city flower here.

Yet again, it's hard to explain why, but this feels more natural to me.

Or, alternatively: "It's a little disappointing that..."

But I do believe that you can see them if you look for them on the internet.

"on the internet" isn't wrong, but you can also simply say "online".