Orchestration and choreography are two basic questions in dynamic Web services composition. In the original orchestration, it can't adapt the dynamic services selection and composition. Choreography lacks security mechanism in open environment. We designed a MAS structure named CSMWC (collaborative structure of MAS for the Web services composition) from a new view of the dynamic Web service composition to remedy these shortcomings, We formally describe the constructed MAS system framework by using Spi calculus and reason the dynamic property, its adaptability, securities etc for the dynamic Web service composition. Finally, we test and demonstrate our idea by the SPRITE tool based Spi calculus.