The US is represented in China by the US Embassy in Beijing. There are a number of Consulate in China such as the US Consulate in Chengdu, the US Consulate in Guangzhou as well as the US Consulate in Shanghai and the US Consulate in Shenyang and also the US Consulate in Wuhan. See their contract details on this website.
You can call the Embassy for assistance with non-immigrant visas however if you are in need of a immigrant visa (K1 Visa and K3 Visa) for the US then you need to contact the US Consulate in Guangzhou for assistance.
US Embassy in Beijing
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Immigrant Visas available only at the US Consulate in Guangzhou
Fax: (86-20) 3884-4412
Visa Information Call Center 4008-872-333 (in China) & (86-21) 3881-4611 (from overseas)
In case of emergencies, please contact (011-86-10) 8531-4000 if you are calling from the United States, or 010-8531-4000 if you are calling from China.