The EFDC model has been used to study response of water quality to varying degrees of pollution reduction in Changtan Reservoir. After verification of the model, a water quality model of Changtan Reservoir was built. Algae growth was simulated under the present situation of pollution load. Pollution reduction scenarios based on two hydrologic years were simulated using the water quality model. The results indicated that overmuch nutrient could lead to eutrophication even in wet years. Under 10% reduction of pollution load, average chlorophyll-a concentrations in three areas (reservoir tail, middle and head areas) decreased by 13.99%, 12.00% and 10.35% in wet year and by 8.42%, 5.63% and 2.10% in dry year, respectively. Under the condition of pollution reduced by 20%, chlorophyll-a concentrations in three areas decreased by 26.78%, 19.25% and 17.04% in wet year and by 11.72%, 7.97% and 5.12% in dry year, respectively. The water in the reservoir tail area behaved flowing-water similar to rivers, while the water in the reservoir head area also behaved still-water similar to lakes, which has been more dramatic in dry year and the average chlorophyll-a concentration decreased by 5.12%.