Effects of PAMs different in molecular weight (MW) (12×10⁶, 15×10⁶ and 18×10⁶ Da) and hydrolysis degree (7%, 20% and 35%) on soil infiltration and soil erosion were investigated. Results show that all the PAMs, regardless of MW, demonstrated pronounced effect on infiltration and increased steady infiltration rate by 58.9%，92.2% and 83.3%, separately, over the control. The effect of PAM medium or high in molecular weight was more significant than that of PAM low in molecular weight, but no big difference was observed between the former two. All the three PAMs decreased soil erosion by 26.3%，52.6% and 26.3% separately as compared with the control. PAM medium in molecular weight was more effective than the other two. PAMs, though different in degree of hydrolysis, all increased steady infiltration rate by 1.9, 2.4 and 2.3 times, separate over the control. The PAM 20% in hydrolysis was more effective than the other two in increasing initial infiltration rate, but they did not show much difference in affecting soil erosion. PAMs low in molecular weight have short chains and hence are unable to form bridges between two adjoining soil particles, thus weakening their function of cohesion, while PAMs high in molecular weight are long in chain and hence unable to penetrate into the space between soil aggregates. However, PAM moderate in molecular weight have chains moderate in length, and can easily go into soil pores and bridge soil particles, thus enhancing their function of cohesion. PAMs low in hydrolysis are low in charge density and in adsorption, too, while PAMs high in hydrolysis are high in charge density, thus forming high repulsion between molecular chains, which in turn leads to reduced adsorption. Therefore, PAMs, 20% in hydrolysis, are the most suitable sort for field application.