Slope usually lies in an initial stress field, especially in complex three-dimensional stress conditions. When horizontal stress is larger than vertical stress or the two horizontal stresses are not the same, the horizontal in-situ stress has significant effect on the excavation slope stability. To investigate the effects of the original in-situ stresses, the strength reduction method is used to analyze three-dimensional excavation slope, and Mohr-Coulomb yield rule is adopted, the failure criterion based on analysis convergence is suggested. First, the horizontal stress is uniform based on the gravity load, when lateral pressure coefficient increases, the safety factor decreases, and the plastic zones develop from slope to foot foundation. Then the condition of lateral pressure coefficient change is analyzed, slope direction pressure coefficient increases, plastic zone on the slope bottom increases, the safety factor decreases remarkably. Using the same method to analyze that of the lateral pressure coefficient on the slope face direction increases, the slope FS increases little holding the K2 unchanged with the K1 adding. The results show that the slope direction pressure coefficient has more effects on the safety coefficient than that of the axis.