In order to study the failure mechanism of expansive rock slope under rainfall, on the basis of a non-linear finite element method considering fluid-solid coupling, the numerical simulation on field test of expansive rock slope failure under rainfall is carried out, the stability analysis technology is improved, which considers the expansion and crack characteristics and material softening characteristic by absorbing water. The numerical result shows the material softening characteristic by absorbing water is the main reason causing the progressive failure process of expansive rock slope, and expansive characteristic can intensify this effect. A shallow crack which is caused by weathering and dry-wet cycle is the source of shallow slide of expansive rock slope.