When you enter the baggage claim area, there is a counter to sell the Beijing SIM Card, you can buy the SIM card here:
The counter to sell the SIM card in the baggage claim area
SIM card of The China Unicom
SIM card of The China Mobile
Beijing PEK Airport To Great Wall of China, Transit Layover Private Tour, by Licensed Taxi, Car Rental with English Cab Driver, Self-Guide Day Tour