Based on the characteristic of GIS Web Service and dynamic Web Services Composition, this paper proposes the implementation mechanism of dynamic GIS Web Services Composition, which can effectively adapt to the dynamic characteristics of GIS Web Services and have vast data and CPU-intensive computing for the concept of Service Group(SG). According to the mechanism, the paper proposes the technical architecture of GIS Web Services composition, providing holistic framework and top-level guidance for studying dynamic GIS Web Services composition technology. Based on the results of workflow research, this paper puts forward Dynamic Service Composition Reference Model (DSCRM), which builds the framework for the development of dynamic services composition system and the development of dynamic services composition application. The ideas of this paper have been used in city emergency disposal project successfully.