The cost of maintain a data center is increasing rapidly, especially for the medium data center. An economic choice is to use cloud computing and cloud storage instead of manage data center by itself. Small companies buy compute and storage service just like water and electronic. The difficulty is how to ensure their data safe in cloud storage. Cloud storage provider claims that they can protect the data, but no one believes them. In this paper, we present a framework to ensure data security in cloud storage system. In the framework, we use SLA as the common standard between user and provider. And we discuss several technologies to make the data stored in cloud safe. These technologies can be divided into three parts: storage protect, transfer protect and authorize.