Foreigners are allowed to work second jobs on the Chinese mainland even if their Employment Permit is registered to another company. The authorities will have no issue with this.

However, if this is something you are looking into, then you should make sure that your current employers will be okay with the situation – if there is nothing specified in your contract about not working part-time jobs, then it is legal. It is also advisable to sign a part-time contract with the company so that your rights can be protected.

If the part-time company is either Chinese or a foreign company based in China, it should deduct the taxable income from your salary first before paying the money to your personal account. This money should then be sent by the company to its local tax bureau. Some companies may pay the full amount to the employee and tell them to pay the money themselves at the tax bureau, but this is fairly uncommon.

If you are being paid by a foreign company based overseas, it should also deduct the taxable income (as applied in its own country) and submit that sum to the foreign tax bureau before paying you your wages.

Calculating tax rates for part-time jobs for foreigners

If your monthly income is 4,000 yuan or less, your taxable income is your income minus 800. The actual tax you pay is 20 percent of the taxable income. 

For example, if your income is 4,000 yuan per month, your taxable income is 3,200 yuan (4,000 minus 800). Of that, 20 percent is tax, so the total tax paid each month is 640 yuan.

If the income is 4,001 yuan or more per month, the taxable income should be 80 percent of your income, and then the tax you should pay is 20 percent of that figure.

So for an income of 5,000 yuan per month, the taxable income is 4,000 yuan, so the tax paid each month is 800 yuan.

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Source：One-Stop