A new methodology of mobile spatial information service is studied based on grid computing environment. Key technologies are presented in the methodology, including grid computing, Mobile Agent (MA) distributed computing, Global Positioning System (GPS), Code Division Multiple Access (CDMA), Transfer Control Protocol/Internet Protocol (TCP/IP), and User Datagram Protocol (UDP). In view of the narrow bandwidth and instability of the wireless internet, limited processing speed and low memory of mobile devices, a new mobile agent based mobile spatial information service grid model is further proposed that has good load balance, high processing efficiency, less network communication and thus suitable for mobile spatial information service. It can provide applications of spatial information grid computing and mobile service, real-time monitoring and scheduling of mobile objects. The theories and technologies architecture of mobile spatial information service grid are built originally from the base, including spatial information mobile agent model, distributed grid Geographic Information System (GIS) server model, mobile agent server model and mobile GIS client model. An application system for mobile spatial information service grid is therefore developed authorship by visual C++ and embedded visual C++. A field test is carried out through this system in Shanghai, and the results show that the proposed model and methods are feasible and adaptable for mobile spatial information service.