Abstract: To investigate the weakening situation of shear resistance index of rock and soil mass after long-term water accumulation in the slope of an open-pit mine, large triaxial shear tests of loose soil and rock are carried out by the consolidated undrained shear method for gravel soil, mudstone and their mixture. The results show that the shear strength and internal friction angle of sand-gravel soil decreases from 38 kPa and 23.2° to 7.4 kPa and 18.8°, respectively; meanwhile, the shear strength and internal friction angle of mudstone decreases from 136 kPa and 27.3° to 7.8 kPa and 20.7°, respectively. The reliability of the test results is verified by back analysis of landslide, and the stability of the existing slope is calculated. The results show that the shear strength of the slope is obviously weakened due to long-term water immersion and weathering, which is not conducive to the safety of the slope. The results can provide a scientific basis for the slope stability evaluation and parameter optimization.