Daihai Lake is an inland and closed lake,this research used the winter and summer conditions of March, July and December, and comprehensively considered the boundary condition of Daihai lake and impact factors such as water surface temperature, density, natural temperature of water surface,atmospheric temperature, relative humidity to structure the formula about warm water drainage to additional evaporation of Daihai Lake.Through calculation, the additional amount of evaporation loss of Daihai Lake result from warm water drainage of power plant in March is 0.3946 million m3, that in July is 0.733 million m3, that in December is 0.106 million m3.There are about 6.782 million m3 warm water drainage to additional evaporation of Daihai Lake every year. The result shows additional evaporation has consistent change with power plant warm water drainage.The research has important significant for the analysis of amount of evaporation to the use of water resources in dry area.