Refer to Apple, and redmondpie.
It’s slightly cheaper to buy iPhone from US and use it in China. The trick is to figure out which one to buy, because not all models can be used smoothly in China. Some of the models are designed towards the US carriers (e.g. the T-mobile version). Here are the recommended models.
Short version
Go for the SIM free version:
iPhone 6 (model A1586)
iPhone 6 Plus (model A1524)
Long version
China has 3 main carrier, China Mobile (biggest), China Unicom and China Telecom. China Mobile has its own standard for 3G/4G implementation (those bands starts with TD), and it’s not compatible with the rest of the world.
Here is more details from redmondpie:
The SIM-free variant of the iPhone 6 and iPhone 6 Plus is the one which is tailored to be used pretty much anywhere in the world, with compatibility for 4G LTE capable carriers in various nations. This model also works on China’s advanced TD-LTE and TD-SCDMA networks, so you can rest assured that all the necessary bands are there when you get hold of the device. The SIM-free iPhone 6 and iPhone 6 Plus may not offer full compatibility with carriers in the United States, therefore you might be restricted to 2G or 3G on some popular networks.
Translation by Google:
请参阅苹果和redmondpie。
这是稍便宜，从美国购买iPhone，并用它在中国。关键是要找出哪些人来买，因为不是所有型号都可以顺利地在中国使用。有些机型都对美国运营商设计的（如T-Mobile的版本）。以下是推荐的机型。
短版
去SIM卡免费版本：
iPhone 6（型号A1586）
iPhone 6加号（型号A1524）
长版
中国有3个主要运营商，中国移动（最大），中国联通和中国电信。中国移动有自己的标准，3G / 4G的实现（这些频段开始TD），它不是与世界其他国家相兼容。
下面是从redmondpie更多详细信息：
在iPhone 6和iPhone 6加的SIM卡免费变种是是专为使用几乎在世界任何地方，用在不同国家的4G LTE能够兼容运营商之一。
这种模式也适用于中国的先进的TD-LTE和TD-SCDMA网络，所以你可以放心，所有必要的乐队在那里，当你弄个设备。
该SIM卡免费iPhone 6和iPhone 6加可能无法提供与美国运营商的完全兼容，因此你可能会限制到2G或3G上一些流行的网络。