Ms.Xie, you still retain your Chinese citizenship although you get the Green Card of USA and your Chinese passport is expired. The Green Card holders is not equal to USA nationality. You can still retain your green card if your Chinese passport is expired. The important point for retaining your green card is not subject to the expiration of your Chinese passport. You'd better not leave USA for more than 6 months. If you do so, the immigrant officals will doubt that if you will give up the permannet residency according to may factors, such as your family ligament (do you have family members or relatives settle down in USA), your economic status, your work, the purpose & time of departure