In the design of suspended diaphragm wall of existing soil-rock foundation pit, the bottom of the wall needs to reserve rock shoulder to provide embedding, and prestressed anchor should be used to lock the foot at the same time. It is the first time to propose a new type of suspended diaphragm wall supporting structure without rock shoulder and prestressed anchor, and makes full use of the inner struts and the middle plate of the underground structure to form a stable system, which lacks engineering experience and theoretical verification. By means of numerical simulation, the deformation characteristics and instability failure mode of soil-rock foundation pit supported by the new type of suspended diaphragm wall were discussed. The research shows that: (1) The deformation of the excavation meets the requirements under the supporting of the new suspended diaphragm wall without rock shoulder. The deformation of the wall mainly occurs in the soil section, and the deformation is mainly caused by the lateral water and soil pressure of soil. The shear stress distribution of the rock section shows that the top of the rock section is not the control point, and there is no need to take additional measures. (2) The method of ground overload can cause the damage of the supporting system components. At this time, the deformation of the rock section is still small, and the safety reserve of the rock wall section is higher than that of the supporting section. (3) By increasing the stiffness of the upper supporting structure to ensure the stability of the upper supporting structure, and increasing the ground overload to study the failure mode of the lower rock section, it is found that the failure mode of the lower rock section is that the shear stress exceeds the allowable shear strength and produces a sliding surface, and the maximum shear stress occurs at the junction of the rock section and the basement.