Based on the observational data of small evaporation pan and theoretically calculated water surface evaporation of 472 meteorological stations in China from 1957 to 2001,the distribution character of conversion coefficient and its tendency during the past 45 years are studied.At the same time the regression model of conversion coefficient is given using different meteorological factors.The results indicate that national average annual conversion coefficient oscillated around 0.622 with a small amplitude.The maximal annual conversion coefficients occurred in the northern of Tianshan,the southern of Shanxi and the Yangtze River valley east of 105°E;while the minimum in Hanhai basin and the northern of the Hengduan Mountains.In the past 45 years,the conversion coefficients of 286 stations were decreasing or increasing,with a change rate within ±0.04/ (10 yr) at 275 stations of the 286 stations.