Abstract: Aiming at the special engineering geology condition in the southwest mountainous area, where there is relatively soft soil with a finite thickness, an optimization design method for a single step loading berm is put forward. Simplified calculation methods of safety factors of various typical sliding surfaces, involved in the optimization design, are also proposed. Furthermore, the optimization design method and simplified calculation methods are proved and verified. Finally, the errors and applicability of the proposed methods are discussed. The verification results show that the relative errors of the methods, put forward to calculate the safety factor or size of the loading berm, are no more than 12% to 20% compared with the finite element method. Moreover, when applying the methods to the engineering geology condition with hard crust or typical sliding surface with loading berm, the relative errors of the methods are smaller than those under ordinary conditions.