Gully erosion depth is one of the most important parameters for the assessment and prevention of debris flows. So far, there is no good method for calculating the gully bed erosion depth of viscous debris flow. This paper discussed the calculation method of erosion depth of viscous debris flow based on theoretical deduction. The formula for calculating the erosion depth was constructed based on deduction from the viewpoint of theoretical mechanics. Then, the affecting factors of gully erosion were discussed. This method can be used to calculate the erosion depth for viscous debris flow under different frequencies and provide a better evaluation and prevention tool for mitigating debris flow disaster.