In order to predict the influence of water storage on plain reservoir, the paper used the finite element software ABAQUS to simulate the stabilize percolation structure of some plain reservoir so as to get the burial depth of groundwater and to determine whether there will be the phenomenon of soil salinization by comparing the depth on the basis of theory of unsaturated soil mechanics. The result indicates two sides: ① The higher the water level of reservoir, the faster the seepage velocity , meanwhile the wider the range of soil salinization in the downstream of related reservoir. ② The reservoir leakage recharges groundwater and heightens groundwater level, it finally reach a steady state with time, meanwhile, the degree and range of soil salinization resulting from water storage of reservoir can also reach a steady state. According to the comparison between the actual engineering monitoring data and the corresponding part of the results in this paper, it can be confirmed that the method is basically reliable and can provide the basis for the prevention of soil salinization around the plain reservoir.