The eutrophication of water body will destroy the balance of ecological environment.To determine the total nitrogen content in water body exactly,is one of the most important evaluating indicators that whether the water body has been polluted by eutrophication or not.After the foundation of the Three Gorges Reservoir,current velocity lowers and it may result in the eutrophication of water body.In terms of the Lambert-Beer Law,the authors adopt potassium sulpersulphate oxidation-ultraviolet spectrophotometry to determine the total nitrogen content in the Three Gorges Reservoir.By analyzing a variety of possible factors,they bring forward a method to determine the total nitrogen which can be used in lab and also have high performance.With this improved method,the authors determine the factual water samples of the Three Gorges Reservoir in the main urban area of Chongqing,and the result is that the total nitrogen content is(0.6~1.0) mg/L,less than the limiting value(2.0 mg/L) in national standard.So the water body of the Three Gorges Reservoir still eutrophication does not take place.So eutrophication does not take place in water body of the Three(Gorges) Reservior.