Abstract: In order to reveal the instability mechanism of waste dump slope in water soaked mudstone base, the damage evolution law of shear strength parameters of weak mudstone layer with water content is explored by indoor test. Combined with numerical calculation software, the response characteristics of water content to displacement, sliding failure characteristics and safety factor of waste dump slope are analyzed. The results show that with the increase of water content in weak mudstone layer, the distribution law of cohesion and internal friction angle decreases nonlinearly; the maximum horizontal displacement, maximum vertical displacement and maximum shear strain of the slope are positively correlated with the water content. The slope is dominated by horizontal movement, mainly showing sliding failure along the contact surface of the waste dump base; the safety factor of the slope is negatively correlated with the water content. In order to ensure the stability of the slope, it is recommended that the water content of the weak layer of mudstone should not exceed 30%.