Abstract: Starting from N and P average concentration,the authors design a predictive model of water eutrophiation in Erlongshan Reservoir on the basis of Dillon model and the intrinsic trait of the reservoir pollution.And with the model,the authors forecast the tendency of water quality and pre-evaluate the water eutrophication.The results show that total N and total P concentrations tend to increase simultaneously in the next 50 years,but the concentrations of the latter will increase much faster than that of the former,and the whole water eutrophication will be aggrvated year by year.Finally,according to the diagnosis model,the authors figures out the reservoir capacity of nitrogenous and phosphorous salts and comes up with concrete options to abate nitrogenous and phosphorous pollutants.