Many world airports have free WiFi and Capital Airport in Beijing is no exception. But don't expect to just attach to the free wifi and start surfing. WiFi in China has to be traceable so the airport in Beijing has machines that will scan your passport and issue a pass like the one I got below. I'll say this for the free WiFi at the Beijing airport, it worked much better than the free WiFi at SeaTac airport in Seattle. The last time I tried to use the free WiFi at SeaTac it was unusable.
