What kind of mobile phone can I buy in China and use in the United States?

Generally speaking, you can use Unicom's 3G WCDMA mobile phone. These WCDMA phones all support 3G GSM and WCDMA, as long as you open international roaming.

If you want to go out for a few days, you can consider renting a mobile phone. If it is an existing smart phone, as long as it is not 3G(CDMA2000), it can be used by the American imperialists. Mobile 3G(TD-SCDMA) can only be reached by 2G.

As for the first frequency of GSM, the second frequency or the third frequency, there is generally no need to struggle. After all, it's antique. Europe and the United States use different frequency bands. If you really want to roam the world, you should buy the iphone 5 5s 5c. Real-world mobile phones can be used in any country.