Modelbase(MB) takes an important role in Water Pollution Control Planning Geographic Information System(WPCPGIS),which endows the system with functions of water environmental simulation,data analysis,predicting,and providing feasible advices.Modelbase(MB) is the focus of this paper,with VB as the programme language,GeoMedia as the system developing platform and Microsoft Access as the database software.At the same time,the WPCPGIS Modelbase is applied in the water pollution control planning of Daxi River in the Three Gorges Reservoir Region,the water environment system relatively is simulated well,provicling planning-maker and decision-maker numerous useful informations:the distribution of pollution source,the situation of water quality,the environmental capacity of water,the situation of water quality in designing year,etc.The result of Modelbase can also be visually displayed in the digital map of GIS,therefore WPCPGIS can be directly applied to the decision-making of Water Pollution Control Planning.