Abstract: The advantages of soil nailing walls are convenient construction and low cost,etc.They have already been used in protective structure of deep excavation extensively.The appropriate design software such as Lizheng Deep Excavation Design Software has usually been adopted in China to design the soil nailing walls.However,it was discovered that the stability of the soil nailing walls was greatly influenced by the calculation depth at the bottom of foundation pits,and the minimum factor of safety of the soil nailing walls could not be really found according to the Lizheng Software interface.The method of the most dangerous slide surface using Lizheng Software was put forward,and was compared with the author’s method.