It is not suggested that you purchase a sim card at the airport, where the goods are expensive. In downtown Shanghai, you could purchase a sim card easily, such as at the 7-11 and hotels. The sim card is supplied by two companies: China Mobile and China Unicom, both of which have set many agents in Shanghai.

Usually, the card is free but you should prepay some money (CNY100~..). Some agent may charge CNY10/card.

Thanks for ur reply. All hotels in shanghai do sell sim card? CNY100 can last for how long? Do they have cheaper 1 like CNY50 prepaid credit? And the reload after that can also be purchased from 7-11 and hotels too?

No, hotels do not sell sim card. There are CNY50 prepaid card and it depends on how many calls to see how long it lasts. It is expensive to make international calls. 7-11 is just open in Shanghai last year. I have no idea if it has sim card to sell. For China mobile, phone calls within the city costs 0.1CNY per minute.