Me neither... Python will never ceases to surprise me. :)

That's not an issue. Your code is too slow. The 6s time limit exists to test performance.
You need to think of a better way calculate the answer.

The number n maybe as large as 1000000000.

Please mark all solution relevant texts as spoilers–even if you post a comment on a solution, and yes, even if you post in Chinese.

By the way, you will get more feedback if you write in English.

That being said, nice solution, although your explanation is a little bit wordy.

You return None if the number isn't a square. For example your is_square(3) is None. But it should be False. This is also reported by the tests:

should work for random numbers
  Your solution was incorrect on 1845030339: None should equal False

(The number will of course vary)