You can buy the GSM SIM card after your arrival in China. Shenzhouxing (Global card) issued by China Mobile and Ruyitong issued by China Unicom are recommended for foreign travelers in China, since they require no Chinese resident document. A SIM card costs CNY100.00. Prepaid charging card in amount of CNY50.00 and CNY100.00 are available in post offices, stores and newsstands, are easy to use with bilingual prompt. If the balance of your card is in shortage, the system will ask you to deposit more money to your card when you make phone calls. Hope it helps.