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If my parents need to file income tax, what documents they need to show to IRS to prove their foreign income (i.e., 退休金) ?

-They may just ask the Financial Departmewnt of their previous work unit (employer) to issue a certifictae indicating their retirement income of the year, and then you can ask a friend to translate it into English.

See this IRS website for foreign retirement income tax return filing:

http://www.irs.gov/Businesses/The-Taxation-of-Foreign-Pension-and-Annuity-Distributions

Many thanks for your constructive comments!