Abstract: Based on the stress states of slope soils and the failure theory of Mohr-Coulomb, "parallel slope-curve" failure model is proposed, which includes the tension zone at the upper edge, the main slide zone in the middle and the squeeze zone at the lower edge. By using the elements of integral method, the logarithmic spiral equation expressing the rationality of the slip surface at the upper and lower edges is justified. The method of "parallel slope-curve" composite sliding is proposed taking into account factors such as penetration of soils and adopting effective shear strength. The researches indicate that under infiltration depth certain conditions, with the increase of the main slide area L₂, the safety factor of shallow stability decreases, and it eventually tends to the same value as the infinite slope stability method. When the infiltration depth reaches the maximum depth , the relevant L₂ drops to zero, and the slip surface degrades as logarithmic spiral characteristics of deep sliding failure mode. When is less than , the whole curved surface method for stability analysis of shallow will produce dangerous results.