這個問題在版上問過，也看別人問過。但是答案不是很清楚。今天問了“tax experts”分享一下。
中國和美國沒有Totalization Agreements， 所以回中國的美籍和綠卡要繼續交social security tax. 我還在繼續研究具體操作的問題，答案找到後再來分享。
“Social Security is one of the areas of taxation which commonly results in dual taxation for expats. This is because US citizens and Green Card holders are required to pay into (and are covered by) Social Security regardless of where they live or work”
The issue with this regulation is that many foreign countries also require individuals to pay into their social insurance systems in order to cover the benefits individuals may receive while living there. This means that many expats are taxed doubly because they are paying into both their host country’s social insurance program and US Social Security simultaneously. To resolve this issue of double taxation, the US has entered into Totalization Agreements with over 20 countries, and these agreements determine which social insurance system an expat pays into. While this relieves double taxation for expats in over 20 countries, there are still many individuals who end up paying into both systems while working overseas“
