What is the average postdoctoral salary in China?

According to this news report Low salaries discouraging overseas academics from ChinaDaily USA,

... the average salary across all ranks and universities is roughly 6,000 yuan ($982) a month. That's low compared with the average entry-level salary in Canada of $5,733 and a full professors' $9,485. The average for newly hired faculty members in the US is $4,950.

Hiring postdocs is still a new trend in China. I can't find average postdoc salary yet. Once I see it (it may be in Chinese), I will report it here.

If average salary in the universities is USD$982 per month, I think I cannot say 40k USD per year is low.

Hope this helps.

You asked about tax as a postdoc in China, whether or not the salary for postdocs is tax free likely depends strongly on which country you are from. Some countries have tax treaties with China which include provisions for researchers to work tax free for a certain amount of time. The university will likely know better, but even then you can't be completely sure they will have up to date correct information.

As of 2016-2017, which is the period I am staying in China, the most common salary range paid for postdocs which I came to be informed of was within 8,000-14,000 RMB per month.

I must say this is highly variable depending mainly on agreements with college and supervisor(s). In reality there is a base salary which is paid by the government directly (in my case of ca. 5,600 RMB) and incrementing on the base salary is the usual practice everywhere. Typically there will be a bonus paid by the college, plus some extra paid by the supervisor, and plus there should be contractual agreements on who should pay for the postdoc's 'welfare' (i.e. loose term concerning rental & bills) and health insurance. In my case taxation happens only on the base salary and has been less than 5%.

Typically larger sums are offered to PhDs coming from universities ranked among top 100-300 in the world. Moreover it is my impression that US-passport holders are offered larger bonuses than PhDs from other countries, often in private.

One should expect some uncertainty on the full amount because a large portion relies on agreements and negotiation. The local culture has it that agreements are volatile and highly dependent on interpersonal relationships, and law/lawyers should have no business in the academia. It is common practice that salary bonuses are used to pressure students/postdocs to do as the college and/or supervisor wishes. Examples: salary deductions as punishments for being late, or caught chatting on the phone during work hours; not adding honorary authors; publishing less papers than expected; damaging equipment or breaking glassware. A bonus may be withdrawn under any pretext after some dispute, including open technical criticism.

The same flexibility always allows for negotiating raises or extra benefits, which are typically in exchange for co-authorships on more papers. These may include covering plane tickets to see family, or travel expenses. Don't be surprised if the raise is pushed further as a bait.

Finally I should stress that at most universities in China there are prizes for publishing papers and depositing patents. The prizes are normally money, calculated based on impact factor. For example publishing in a mainstream academic journal with IF 2-3 in the Life Sciences may be awarded 4-7k RMB. Publishing in top impact journals may award the first author much more (also whether the award should be shared is negotiable).