Abstract: The annual evaporation of the wet region here is defined as the sum of the evaporation from water, soil and the evoportranspiration from plant in the study region. According to the water balance principle, for a closed basin, the annual evaporation can be written as:E_i=P_i-R_i±ΔW_i(1).In which ΔW_i is the difference of the water storage of the basin. 1 proposed to solve the above equation by seperating the soil into different zones.I found that, from the analysis of three factors of annual water balance in the wet regions of Fujian province, land evaporation is quite stable from year to year and its covariance C_v=0.09. I also found that, from the analysis of the presentative of the mean, if the mean of a short series has a good presentative then it can be used as the mean of long series.From the fact that the annual evaporation in the wet region is quite stable from year to year, by study I suggest the following equation to calculate the statastic parameters of annual runoff.