多謝8老師的解答！所以說，如果在中國每次待的時間不超過6個月，比如每隔5個月來美國待一個月，這樣弄幾年以後，有可能因為沒有每年6個月以上的時間呆在美國，導致綠卡被海關或者USCIS收回或者不能renew嗎？
-That will be trouble. Your parents must live in US physically for more than 6 months every year to keep their green card. Also, they cannot go back and forth between US and China very often, e.g., live in US for two months and go to China for two months; then go back to US for two months and leave again, and so on.
As I said, the green card is for them to live in US permanently, not use it for border crossing card. If the immigration officer considers they do not have the intent to live US permanently, their green card will be revoked.
再次感謝！
