Stability analysis of excavated high slope is a complex problem. This work aims to a real excavated high slope in argillaceous mudstone, the strength properties of mudstone and instability mechanisms of slope had been studied separately by laboratory tests and numerical stimulate. The test results of residual-strength under large strain and softening-strength under rainfall infiltration were presented. The stability estimate basis of Bishop circular arc method and non-linear stress-strain analysis based on Duncan constitutive model had been carried out on the slope, using different strength indexes and Duncan model parameters obtained by triaxial drainage shear test. The results show that the landslides mechanisms caused by excavation and rainfall infiltration are relaxation failure and soften failure, separately. The deformation zone lies within the 0-4m soil layer and slope toe. The reinforcement measures of resistance piles set at slope toe and combined bolting and shotcrete set at slope surface were presented. After reinforced, the stability of excavated slope increased.