Environmental problems have been modeled and analyzed using GIS for long time, and they are often described by a set of interacting models from multiple disciplines. In this paper a composite process for environmental problem with multiple models is analyzed. A system dynamics modeling method, STELLA, is introduced to illustrate how to model the composite process with cause-effect relationships. Compared with other environmental modeling methods, environmental modeling supported by semantic geospatial Web service has the advantages of achieving interoperability and allowing real-time and online updating of the deployed models. A prototype system for Modeling Environmental Process using Semantic Geospatial Web Service, i.e. MEPSGWS, has been designed, which includes process ontology, rules for cause-effect relations, GML-based parameter type, Web service description language, and interoperable protocol. As an application of MEPSGWS, a watershed composite process is chosen to illustrate how to model a composite environmental process with semantic geospatial Web service.