A two-dimensional, laterally averaged Model CE-QUAL-W2 was applied to the Shanzai Reservoir to simulate the hydrodynamics and water quality in the water column. The modeling effort was supported with data collected by fieldwork and experiments within half a year. Furthermore, the study made a water quality assessment of the eutrophication in the outlet of reservoir from Apr. 7th, 2006 to Jun 7th, 2006, using Trophic State Indexes (TSI) method and analytical hierarchy process(AHP). The result showed that, On May 12th, TSI was higher than the other days, almost reached 80; and the first big water blooms lasted from Apr. 29th to the beginning of June, 2006. The modeling result of three scenarios (keeping original flux, decreasing water level and increasing flux) indicated that decreasing water level and increasing flux could greatly improve the water quality of outlet．