While the outsourced storage services bring economy and convenient, it also brings a lot of security problems, especially data security. In recent years a lot of research have done on the protection of data security in outsourcing storage services, most of them focus on the data privacy and integrity protection, and pay little attention to the guarantee of data freshness. The data freshness guarantee means that data must be kept as the latest version whenever user access it. A freshness guarantee mechanism for data in outsourced storage services is proposed. This mechanism guarantees freshness effectively on the basis of data privacy and dynamic access control.