In this study, a hybrid GIS-supported watershed modelling system is developed for integrated planning of water pollution control in the Lake Erhai basin, China. The system comprises a GIS-supported database, simulation/optimization models, and user interfaces developed through Arc/Info and Arc View. An inexactfuzzy multi-objective optimization model is developed for assessing a variety of system objectives and generating desired decision alternatives. It can effectively reflect uncertain, interactive and multi-objective features of the study system, and its solutions provide decision makers with a flexible decision space. The GIS technology is used for managing spatial and non-spatial data, linking models, and providing interfaces between the models and their users. The developed modelling system allows regional planners to obtain desired plans of human activities under a variety of objectives and constraints and to examine consequences and impacts resulting from alternative management practices.