Abstract: In view of the fact that lithology deterioration condition leads to the decline of slope stability，and with a slope of open-pit mine as the research object，the internal defects of landslide were detected by SIR-20 geological radar，and the change law of slope stability under the condition of different lithology was analyzed with Slide software.Results showed that：slope lithology deterioration is caused by the interaction of weak interlayer，the rainfall infiltration and the blasting dynamic load，which lead to the slope sliding and instability in the late; c value reduction is the main mechanical performance in the process of lithology deterioration; Safety factor and c value showed as a partial parabola relationship，and the reduced c value 50 kPa is the critical value of slope instability under special working conditions.According to a comprehensive analysis of mine operating characteristics and the lithology deterioration condition，a proposal to improve its permanent stability by decreasing the upper quaternary bench slope angle and final slope angle is put forward.It is proven that the safety factor could meet the requirements.The results provide an important basis for safety mining and determining slope control parameters in the late.

Key words: Open-pit mine, Slope stability, Lithology deterioration, Numerical simulation, Cohesion, Safety factor