Nowadays, Web services are usually aggregated into a composite one to satisfy customer?s more and more complex requirements. Generally, there may be several different candidate services to carry out one task in a composite service, so a choice needs to be made to help users select the most suitable one. Based on the quality of services, this paper generates a Weighted Multistage Graph for composite service, and transforms the problem of service selection into the one of how to get a longest path. Considering the problem of Interface Matching, this paper presents a 3-layer Web service organization model (WS3LM), which can help get an executable composite service. This paper describes and compares two types of selection approaches: one type of local optimal selection and the other type of global optimal selection using Exhaustive Search Algorithm, Dynamic Programming.