Abstract: In this paper, a series data of monthly evaporation (1974～1991), in Jiangsu province’s northern coastwise protective forest system which has the characteristics of moist maritime climate, are calculated by appropriate method from the physical regularity in evaporating. Then, according to the analytical results of monthly evaporation by comparing in shelter belt region with in other regions both on time and space, the variation of effects on evaporation according to growing of forest is obtained and it is confirmed that Jiangsu province’s northern coastwise protective forest system has an active effect on regulating the climate.