Synthetic materials of polyacrylamide (PAM) are known as the flocculating agent as well as water retention agents. In this study, ten types of water-soluble PAM as well as four types of water-insoluble ones were selected and the influences of relative molecular weight, ion types, charge density and particle size on water retention and service life were determined. Based on the results, evaluation method for performance of water retention agent was established and two optimal PAM (water-insoluble JB and water-soluble WSN20) were screened for further study. It showed that JB increased the degree of hydration of testing soil for 32% compared with that of control. Moreover, multiple-step-outflow test using municipal waste showed that addition of JB (0.1%) had significantly effect on its moisture characteristic curve as evidenced by increasing of equilibrium moisture content over 12% under high matrix potential.