For the stability analysis of cohesive soil slope with weak interlayer under condition of heavy rainfall, a new method is proposed based on the slope investigation data and the fleory of unsaturated seepage and shear strength to analysis the seepage characteristics and variation of safety coefficient for soil slopes containing weak interlayers. Analysis results indicate:due to rainfall infiltration, the transient saturated zone is formed in the weak interlayer of slope; Under rainfall infiltration process, there is positive correlation between the suction and the vertical effective stress; With increase of rainfall duration, the plastic zone extended from the inside interlayer to the top of slope, as a result of which the slope safety coefficient decreases; After rain, weak interlayer will still exist in local transient saturated zone, causing less significant decrease of the slope safety coefficient. According to the numerical results, the instability process of cohesive soil slopes can be divided into 5 stages:interlayer softening,interlayer extrusion,tensile crack,slope crest settlement and fault slip. Consequently, to minimize the influence of heavy rainfall on the slope stability, reasonable drainage system should be emphasized in the design of cohesive soil slope with weak interlayer.