GIS soft and hydrological model are not always accessible to decision-maker and the public. In order to make GIS-based hydrological model accessible and improve the efficiency of the management of the watershed, a system integrated with web and distributed hydrological model is design and developed in this paper. Our Web-ESSI system is implemented using C# as programming language over an ArcGIS Server platform to display and manipulate spatial data and using ESSI model to simulate the hydrological process. Web-ESSI provides a good tool to simulate and predict runoff processes on-line.