With the rise of cloud computing, virtualization technology application is becoming more and more prevalent. How to manage them efficiently and economically has become an important problem. This paper proposed an approach which optimizes the placement of virtual machine images based on QoS. It analyzed the revenue of the storage service constrained by QoS and optimized the strategy of images placement. Experiments show that this approach can meet the constraints of user's QoS and improve the revenue of storage effectively.