Abstract: Rainfall infiltration not only changes the seepage field of the slope, but also reduces the soil strength, which is easy to induce landslide. The soil in the slip zone often shows strain softening during the landslide process. However, the research of the two-parameter strength reduction method based on the strain softening model is still in its infancy, and the influence of seepage field has been ignored when analyzing the stability of landslides in the previous studies. The red-brown soil in the Liupanshui area has strong water retention and obvious strain softening characteristics, and many large and medium-sized landslides have occurred in this area. Understanding the disaster-causing mechanism according to the characteristics of red-brown soil is conducive to the prevention and control of landslides. A three-dimensional geological model was established for the Zhoujiapo landslide in Jichang town, Liupanshui city. Based on the fluid-structure interaction theory of unsaturated soil, a mathematical model of the interaction between the flow field and the stress field of slope was established. The rainwater seepage and the stability of the landslide were simulated by using the finite element and the two-parameter strength reduction method considering the strain softening of the soil. The results show that after rainfall, the rainwater mostly flows into the lower edge and gully along the slope in the surface soil in the tangential and transverse directions rather than all vertical infiltration. The saturation at the bottom of the gully increases significantly and the intensity decreases the most compared to other areas at the upper edge. With time extension of rainfall, a plastic penetration zone along the gully is formed in the slope, which greatly reduces the stability of the landslide. Compared with the classical reduction method, the stability coefficient obtained by the double reduction method considering strain softening in the rainfall process changes from small to large, but the difference was within 4%. It can be seen that the negative impact of red-brown soil strain softening on landslide stability is not significant and weakens with rainfall. The prevention and control of such landslides should focus on reducing the permeability of gully soil. This study provides a theoretical basis for the prevention and control of similar landslides.