这个问题在版上问过，也看别人问过。但是答案不是很清楚。今天问了“tax experts”分享一下。
中国和美国没有Totalization Agreements， 所以回中国的美籍和绿卡要继续交social security tax. 我还在继续研究具体操作的问题，答案找到后再来分享。
“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“
