The state of eutrophication of Changshou Reservoir has been evaluated using Garlson model and synthetical nutrition state index. The results reveal that the water quality at all monitoring sites have become eutrophication. The study on the balance of N and P indicate that the total input amount of N is 7 914.6 t/a, 70% of which are from the upper-stream, and the P input is 591.84 t/a, with runoff being the main source, accounting for 47.0% of the total; The outputs of N and P are 2 419.5 t/a and 120.2 t/a, respectively, suggesting that the rates of retained N and P reached to 69.4% and 79.7% respectively. In order to prevent the aggravation of eutrophication, measures have to be taken to decrease the input of N and P from the upper-stream of Longxi River.