Based on the research of properties of unsaturated soil and its mechanism effect on dam material strength,the paper combined the actual project and relied on geo-studio software to compare the influence of saturated soil and unsaturated soil on seepage and stability of dam body. The result showed that in the case of unsaturated soil, the calculated maximum penetration rate is relatively small, the seepage discharge of dam decreases, the safety factor of slope stability increases,which is more in line with the actual situation of dam.At the same time, in the condition of unsaturated soil,it used limit equilibrium method to compare and analyze the factors affecting slope stability. The impact of friction angle and bulk density on safety factor is larger than that of cohesion;the movement of impervious wall forth and back has a certain impact on downstream slope. The results can provide scientific basis for the optimum design of dam and the operation and management of engineering.