To interpret water quality responses to pollution loads and quantify the concentrations of pollut?ants in rivers,a method of water quality simulation model (WASP) integrated with geographical information system (GIS) was presented and a case study was conducted in the catchment basin of the Jiaokou Reser?voir in Ningbo, China. The hydrologic model in AcrGIS and a digital elevation model(DEM) with a spatial resolution of five meters were applied to generate the river hydrological networks and the ranges of basins and watersheds, then the river networks of the of the study area were divided into 168 simulation units. The spatial and hydrological characteristics data of the simulation units from GIS, along with the data of pollutants loads, flows, river hydrologic parameters, water quality parameters and boundary conditions, were converted and integrated to WASP. The spatial distributions of water quality variables in Jiaokou Reser?voir Basin were simulated, and the computational segment data and water quality output data from WASP were combined with GIS geodatabase to visualize and statistically analyze the results. The simulation results show that the calculated values of chemical oxygen demand, ammonia nitrogen, total nitrogen and total phosphorus agree well with the measured values from 6 sampling stations, with the average relative devia?tions of 10.4 %, 18.7 %, 17.4 % and 19.9 % respectively. The average relative deviations of the four vari?ables between the daily simulated values and measured values from the sampling station ahead of reservoir gate are 7.5 %, 13.3 %, 8.6 % and 18.6 %, respectively. The GIS-based water quality simulation method can be applied to study the spatial and temporal distribution of water quality in Jiaokou Reservoir catch?ment basin, and the case study confirms the applicability and efficiency of GIS-based water quality simula?tion in water quality management decisions.