Abstract: In this study，two methods of out-of-plane stiffening are introduced，one is reinforced concrete plate stiffening，and the other is rigid plate stiffening. The shear strength，stiffness，ductility and energy dissipation capacity of stiffened and unstiffened slit walls under shear loading are studied by means of nonlinear finite element analysis using ABAQUS software. Large-deflection effects and out-of-plane initial imperfection are considered. The effect of two stiffening methods on the out-of-plane buckling and stress patterns of slit steel panels are also investigated. The analysis results show that using reinforced concrete plate as a stiffening panel can effectively prevent the elastic buckling as well as development of diagonal tension field action，and significantly increase the shear strength，ductility and energy dissipation capacity. Considering the brittle shear failure of concrete itself，to better restrain the out-of-plane buckling of slit steel panels，it is suggested that the reinforced concrete plate should not be engaged in resisting the lateral-force from early stages.