On February 16th, Qin Baoqing, a villager in Zhangshan Village, Changgou Town, Jining, Shandong Province, caught a wild turtle weighing 25 kg and 70 cm long by chance while burning incense to pay homage to the fishing boat.

For symmetry, I suggest either weighing 25kg and measuring 70cm" or "25kg in weight and 7cm long"

Also "by chance" implies that he was not actually trying to catch turtles; is that really what is meant?

My old lady is more than 90, and it is the first time she has seen itone."

"it" would refer to this particular turtle, meaning she has not seen this particular turtle before. But the prior clause said that he has never seen such a big one, so I think probably you meant that she has also never seen such a big one.

At the scene, someone bought turtles froma purchaser offered Qin Baoqing at a high price, but Qin Baoqing declined and decided to release the turtle back to nature.

I don't know what is meant exactly. I am not sure if you mean someone bought some turtles from him, but he declined to sell the large one, or if you mean that someone offered to buy the big one and he declined to sell it.

"Released to nature" is okay, but "released back to nature" is more appropriate, I think, as it was in nature not long before.

Under the witneseyes of dozens of villagers, this "turtle prime minister" was put back into the lake.

Or "In the witness of dozens of villagers..."