The traditional service model of cloud storage is that the service providers supply both the storage capacities and data storage services through the Internet to the clients. The obvious disadvantage of this model is that the data of the same customer stored in multiple cloud storage providers cannot interact with each other. This paper adds a new layer to this model. The new model consists of StaS (Storage as a Service) User, StaS Provider and Cloud Storage Provider, which can solve the above problem. It also possesses some other advantages, such as Cloud Storage Providers need not care about the market and how customers use their services, different customers can use the cloud storage services in different security levels as they need.