Abstract: In order to investigate the variation in strength of the breccia soil in the Huanxipo outwash deposit under the action of rainfall, in-situ shear tests of the breccia soil are conducted under the natural condition and under four different rainfall duration conditions. The unsaturated strength characteristics of this special soil are discussed by combining the soil water characteristic curve. Rainfall has a significant effect on the shear strength of the breccia soil in the Huanxipo outwash deposit. When it is approximately saturated, the cohesion and internal friction angle are 64.3% and 22.5% respectively lower those in the case of natural water content. The infiltration of rainfall rapidly reduces the matric suction of the breccia soil, and the heavy rainfall in a short time will lead to too much loss of the shear strength of the soil, which is liable to induce the landslide of the shallow soil. The equation of the relationship between the unsaturated cohesion and the water content parameter χ(w)of the breccia soil in the Huanxipo outwash deposit is established. In practice, the polynomial function can be used to determine the parameter χ(w).