Web systems are self-descriptive software components which can automatically be discovered and engaged, together with other web components, to complete tasks over the Internet. Unified Modeling Language (UML), a widely accepted object-oriented system modeling and design language, and adapted for software architecture descriptions for several years, has been used for the web system description recently. However, it is hard to detect the system problems, such as correctness, consistency etc., of the integration of Web services without a formal semantics of web services architecture. In this paper, we proposed an approach to solving this issue by translating the UML web service architecture description into a formal modeling language -SO-SAM, and verify the correctness of the web system design using model checking techniques. We presented this approach through an imaging processing scenario in the distributed web application.