Some problems in the application of spatial information, such as distributed enormous data, heterogeneous format, complexity of processing and intensive computing have been obstacles to realize wide sharing and integration of spatial information. By taking grid technology and web services as its technical core, spatial information grid (SIG) shares and aggregates the distributed enormous spatial information resources to implement dynamic integration and collaboration. Based on building open seven-layer application architecture of SIG, we put, in the research, emphasis on SIG service composition. A novel service composition model named service/resource net (SRN) is proposed based on Petri net and graph theory. Moreover, an innovative service selection model (SISM) is presented to implement the dynamic service selection in the process of SIG service composition. In accordance with the application requirements of the national geological survey, an application example of ground water evaluation is given and the validity and usability of SIG are verified.