It all depends on where you are, but generally speaking, the risk of catching malaria in ANY part of China is relatively low. It's higher risk in less developed places such as Cambodia, Vietnam and parts of Philippines etc.
However, there are some very rural parts of south China where you can still be at risk. Ask your doctor before you go (they'll have better knowledge) and there are some websites out there that can provide 'malaria maps' indicating high, medium and low risk areas.

There's also a link (below) with tips on how to keep mosquitoes away, using a combination of natural and chemical techniques.
Also, if you do decide to take malaria tablets, you have to be careful of some malaria medicines: I've heard they have awful side effects.

What Ludovico said. Malaria is rare in China, usually due to the health checks all children are required to have yearly and the innoculations given. On the other hand besure to becareful of Hep. A and B which is wide spread in the Southern Area.

I lived in the south of China for years. Never got it and was never vaccinated against it. From Yunan to Guangxi, Guangdong, Hainan and Fujian, I have never heard of a case with any expat.