Quantitative calculation of deposit slope stability, which is closely related to the strength parameters and slip boundary, directly determines the cost and planning of the construction. Different from that of the sliding zone, the determination of strength parameter and slip boundary involves great complexity. The deposit slope with neither sliding zone control nor adequate inversion condition is explored. In the study, a deposit slope on the left abutment of a hydropower station in downstream Dadu river is taken as a typical case. Detailed geological data and slip boundary searching by maximum shear stress method show that the potential sliding boundary is formed through the gravel mingled with soft silty clay with unfavorable properties in the middle stratum. The strength parameters are determined comprehensively by engineering analogy, large-scale indoor test and inversion method. Final results reveal that quantitative calculation based on geological data is consistent with the qualitative analysis.