回複：If you got this "Stamp"

US consulate or embassy, you don't need visa again. If you got it from US-Canada border, you still need visa stamp from China. Because USCIS and State Department are different system. Visa is issued by US consulate/ embassy under State Department and used for entry US. USCIS is responsible the immigration status, i.e. whether you are with legal status, only valid in US. I don't know what kind of stamp you have, seems not a visa stamp.