why China is mixed economy?急!!

A quick answer:

Because China has opened up to the world, no longer an iron country -- although it's still ruled under Communism, it accepts trades internationally, promotes tourism, encourages firms to develop, and no longer produces everything in state factories... People work for private firms in order to earn a higher salary (in Communism where everyone is equal in theory, the 'income' of the civilians will be mainly food). Workers pay taxes to the government. They have freedom of choices e.g. choosing which brand of bread to consume.

And so much more. Try to think of some for yourself. You may find it easier if you have found the definitions and pros and cons of Communist economy and Socialist economy first.

Nowadays, even in extreme cases like North Korea (ruled under extreme Communism but realizes the benefits of tourism therefore has opened up a tourist zone) and Hong Kong (rated the freest market in the world, something like over 90 out of 100 points, yet there are many areas where government intervenes), all economies are mixed economies.