Using pre-exist Web services to Provide composed Web services according to the QoS requirements of clients is still an urgent problem to be solved. In order to solve this problem, firstly, we enhanced the current WSDL to describe the QoS properties of Web services and then gave a way to choose the proper pre-exist services based on their QoS properties. Then, an evolved Web Service frame is proposed by adding a new role to the current Web Service frame to realize the QoS driven composition. Finally, we design and implement a prototype, which can flexible select and compose Web services at run time.