Whats the average salary (inclusively) in china? How expensive it is for a single postdoctoral life? I'm going to join one of the top university in China. I have been offered 130000 to 180000 RMB per year (including the basic performance and housing subsidy) on invitation letter although it's less than what was committed with me in the mail. How expensive is Bejeing? This is my third postdoc. I am afraid if it will be lower than the average postdoc. How is the working condition in Tsinghua and Peking University in Engineering Science?
I was a postdoc in China for the latest two years. Mainly at the south of China but have also spent several month in Beijing with CAS. I was hired with a salary promise of annual 180000 rmb which wasn't paid correctly. So actually my first piece of advice is, take salary promises with two grains of salt, and perhaps consider just going elsewhere.

Now, answering your questions directly. I have been told that a Chinese postdoc gets around 9-10k rmb per month in Beijing, and I was offered another postdoc for annual 240k rmb (some special offer for foreigners I refused). This is probably the range you will find. Housing costs in Beijing are extremely expensive (at least 2.5k per month, above 4k if you want to be comfortable) however ordinary life costs are not bad. If you're not provided a place to live, don't accept anything below annual 150k rmb. Make sure you're provided decent healthcare coverage, because that'd also very expensive and unfriendly to foreigners.

Finally, you say this is your 3rd postdoc. Meaning you're probably hoping to get a permanent job soon, and possibly heard promises of a fixed position "in case things go well". Don't fall for that, this is usually bait. From asking several colleagues, they were all promised "something else" when in reality it's all about them distributing around authorships and impact factors until they get fed up.

A tip: Chinese academia generally likes US citizenships and top-prestigious journal authorships. If you don't have any of such to show, don't expect the best treatment.

My main advice remains, consider going elsewhere. Hong Kong is already better if you're into Asia, perhaps Taiwan.

Best of luck.