Why does my phone's GPS not work in China?

It might because China blocks the (Google) SUPL server that is needed for AGPS. A recent story says that Xiomi devices (even those with a global ROM) use the China Telecom SUPL server.

Many good answers here already. But I have been to China before, and used GPS with a very good precision, and with Google Maps. Also, dealing with the coordinates and mobile device GPS is part of my profession, so let me pitch in.

GPS satellites are nothing more than a satellite network that spreads a signal of a timestamp that mobile phones or any GPS chip can catch, and then figure out the longitude, latitude, and the altitude. It works anywhere in the Earth surface unless it’s jammed.
In addition to GPS signals, mobile phones can use the location data provided by the mobile carrier (GSM, 2G, 3G and LTE) to assist in this.

I do not have any references, but China does not block GPS signals. It does however block Google, which can prevent your phone’s Google Maps app from showing your location on a map. Accessing Google services in China is somewhat troublesome. Gmail, Google search, and many other services are blocked. Google Maps, however, worked very well in Beijing. China’s firewall is based on IP addresses and host names, so it’s possible that I was lucky to access it somehow.

The reason why you can find your location when you are connected to a WiFi network is because your phone can use triangulation to determine your location. It just uses nearby wifi network names and query Google/Apple services to get the location, which would have been recorded before through other people. You do not need to connect to any network. Your phone can get the signal strength (measured in dB), and figure out the location.

What you really need is a good offline map, because Google services are inaccessible. I have had a lot of success with Open Street Map (iPhone/Android: “City Maps to Go” app can download them for you, and provide GPS pin pointing feature on the maps). Works very well in China.

For better GPS connectivity, try to get your phone outdoors and keep a direct line to the sky. There is a lot of fog/smoke condition in China, specially Beijing. It’s just factory smoke and nothing with iron particles (which can counter act signals). Keep your Wifi turned on. If you are nearby any public wifi hotspot, there is a very good chance of finding your location quickly.

Long ago I took a fairly simple GPS (coordinates only, this was before the idea of a GPS with a map existed) to China–and found it couldn’t cope with all the tall buildings. In a large park it would work. Once I managed to get a fix with it sitting on a windowsill high up in a building but at street level with buildings around I never got enough satellites for a fix.

Download Baidu Map. Or Autonavi. They are both free softwares, and has English version for foreign tourists. I am Chinese.

Vince covers the likely explanation, but unfortunately there are several more sinister possibilities as well. Long story short, China’s legislation on GPS is both really vague and in part secret: by some readings of the law all use of GPS devices is technically prohibited, and not a few cameras and other GPS-enabled devices go so far as to disable GPS entirely if they realize they’re within China’s boundaries.

Now to be clear here, disabling GPS like this is entirely up to the manufacturer, China itself does not block or attempt to block GPS as far as I know. And with something on the order of 1.2 billion phones in use in China, the vast majority of them with GPS enabled and heavily used by locals, any notional prohibition on GPS is pretty much a dead letter — although it’ll be interesting to see if China’s version of GPS, Beidou, is made mandatory once it launches.

Even if you do get the GPS working, you may find that your favorite Western mapping service doesn’t show satellite imagery in the right location, with imagery instead “randomly” shifted. Chinese services like Baidu and Sohu, on the other hand, will work fine.

This may be due to the long Time To First Fix. Indeed, I recently learned that recent smartphones use mechanisms called “Assisted GPS” in order to get a location based on GPS reception. The idea is that if you use your GPS in an area you haven’t been before, the GPS would take some time to find and interpret the signal, mostly because of the slow download speed from a satellite (the source of the Wikipedia article mentions 12 minutes to get an entire navigation message, to locate the device).

To solve this issue and make it more usable for today’s impatient phone users, phones use the data network (WiFi/mobile) in order to approximate the location of the phone and download the almanac of GPS satellites positions. With this information, the GPS signal is more likely to be found faster with a better precision. As @AdamDavis details in his comment, the technologies providing this solution may not be the same in every country, or even be available.

So your problem might just be that you are not waiting long enough for the phone to locate itself. I would suggest to try to use the GPS feature while connected to WiFi, or just be patient.

Note that anyway, China cannot block the GPS signal. As some commenters say, it might definitely be possible to use jammers to alter (not block) the signal, but I suppose there would be a need for a lot of jammers to cover the whole country. And I am not sure it is possible to jam these signals without altering, at least partially, other telecommunication signals. The GPS signals are sent by US Air Force satellites, it might be possible that they can control finely in time what satellites send what signals and with what precision, but again I doubt of that (the communication with the satellite might be too slow to offer such fine configuration, if it is even performed). As some commented, it is hard to even configure that for a specific area on Earth as the satellites are not geostationary (I.e. a satellite will pass over multiple continents within a day). I am no expert in that field, though.