[Objective] To improve the existing evaluation methods on initial conditions and understand the mechanism of rainfall infiltration on slope stability, in order to provide effective suggestions for landslide early warning.[Methods] Taking Liaoji landslides in southern of Tianshui City, Gansu Province as a case study, soil was defined as a natural or wet state based on whether there was a antecedent rain. The Boltzman model was applied to turn pore water pressure into water content. And the fluid-solid coupling models were established to make a numerical analysis.[Results] In a natural state, soil absorbed the rain rapidly, and infiltration were confined in the superficial layer of 1—2 m. The stability coefficient decreased significantly in the early stage, but slowed down with the stagnation of infiltration, and formed shallow landslides ultimately, similar to plastic sliding. In wet condition, soil held water slowly, but the infiltration reached deeper position(3—4 m) and increased its pore water pressure. As the increase of infiltration depth, the process of soil softening became stronger and stronger and the sliding surface was often occurred in the deep position. When the soil was dry, the 7-day rainfall warming was an effective method for disaster warming. While in the rainy weather, soil kept in wet, and the pore water pressure blow 4 m and the slope displacement should be monitored.[Conclusion] The initial conditions of soil have significant impact on the process of unsaturated seepage and stability changes. Consequently, it is an integrant factor in stability analysis of rain-induced landslides.