Chenghai Lake is situated at some 20km southwest of Yongsheng Town on Yunnan Plateau.The lake area is 77.22km²,its average water depth 25.7m and water volume about 19.87×10⁶m³. Chenghai Lake, a first known slight salty water lake in Hengduanshan Mountains.According to the chemical analysis, the mineralization of the lake water has been gradually increasing, from less than 900 mg/L in the year 1972-1974> to 956-991 mg/L in 1975-1979, and to 1004 mg/L in 1981.The mineralization has an increase of about 10.4 mg/L averagely.The lake water changed from fresh to brackish.The content of main ions in the lake water is HCO₃^- >C0₃^2- ≥Cl^- >S0₄^2- and Na⁴ >Mg²⁺ ≥K⁺ > Ca²⁺, belonging to the type of C₁^Na.With abundant dissolved oxygen and sufficient nutrient elements in the water, this lake has favourable conditions for fishery.As the lake is getting salified successively, Chenghai Lake area is unsuitable for the development of industry and agriculture.Being a closed lake of high mountains, according to the proximative analysis and calculation, Chenghai Lake has a total inflow of 1913.0×10⁶m³ and a total outflow of 2056.3×10⁶m³, correspondingly the salty composition carried into the lake is more than that of the outflow from the lake, hence resulting in salty composition accumulation and an increase in water mineralization.In order to prevent the lake water from the continueons increase in mineralization, it's necessary to increase the surface runoH volume flowing into the lake, to control and raduce the water quantity for industry and agriculture; and to study the problem of restraining the evaporation from water surface.