原因是在中国超过半年.
-I think you may have often gone to China back and forth several times and stayed there for > 6 months recently, otherwise you would have not got the such warning just because you stayed in China for > 6 , 1 year months one time.
因为特殊原因还得回,返程票已定好来不及办白皮书,下次回来会被没收绿卡吗?
-It probably you will have trouble when you return if you do not get the Return Permit, but you still can go to the immigration court to defend your GC if the CBP officer 没收绿卡.