Slag water surface evaporation phenomenon exists in the system of water cooling slag removal. The authors study the relationship between slag water (which is from the JINJIE power plant in Shaanxi)surface evaporation rate and the relative humidity(40%~90%)and the water surface temperature(35-75 ℃),which at an atmospheric pressure and the wind speed is 0.5 m/s,and then obtains an empirical equation. The experiment measures three kinds of concentrations of the slag water surface evaporation rate,and compares the measuring results with those of the pure water surface evaporation rate at the same experimental conditions. According to the comparison results,the authors analyze underlying causes of slag water evaporation from the thermodynamic point of view,that is surface temperature and composition and content changed the surface free energy,thus affects the slag water surface evaporation rate. It is found that water surface evaporation rate is 1.1~1.2 times of the rate of slag water when the temperature is over 50 ℃,while the water surface evaporation rate is slightly higher than that of slag water when the temperature is lower than 50 ℃,and the slag water surface evaporation rate changes in 10% in a certain concentration range.