Goalkeeper Nicole Barnhart, starting for the injured Jill Loyden, made a save to spark a counterattack that forward Sydney Leroux finished off in the 14th minute.
Defender Ali Krieger doubled the lead in the 32nd by scoring her first international goal from a cross by Alex Morgan.
The U.S. extended its unbeaten run over China to 21 straight games when China's Wu Haiyen scored an own goal in the 47th before Christen Press and Whitney Engen added two more.
The Americans face former coach Pia Sundhage when they play Sweden on Monday in the final match of the group phase.
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