The paper established 2D coupling model of water quality and water quantity based on the finite volume method,and analyzed the influence mechanism of exogenous pollution on total nitrogen and total phosphorus of Shanmei Reservoir from August to October 2010.The results showed that it cost 20 days for the exogenous pollution to spread from outside of the reservoir to the whole reservoir by transportation and diffusion, and the water quality of reservoir also dropped from Ⅲ class to Ⅳ class, even inferior Ⅴ class correspondently;the total nitrogen and total phosphorus concentration of areaⅠincreased by 0.16-1.32mg/L and 0.12-0.34mg/L,while areaⅡhad little influence of exogenous pollution,that of areaⅡjust increased by 0.05mg/L and 0.075mg/L;the total nitrogen and total phosphorus in different areas from reservoir entrance to reservoir exit changed greater and showed a decreasing trend and presented certain lag in time.The results indicated that the water deterioration of reservoir is mainly caused by exogenous pollution in a short time.Last, the paper put forward corresponding engineering measures to improve the water environment of reservoir basin in accordance with combining the situation of water environment in Shanmei reservoir basin.