Abstract: The limit value of the stiffness of vertical boundary elements (VBE) in steel frame-thin steel plate shear walls (SPSWs) is derived based on the thin web plate girder theory. The formula works only for the SPSWs with steel frame, but not for the one with concrete-filled square steel tubular (CFST) frame. The formula for the limit value of stiffness of the vertical boundary element of CFST is put forward based on the thin web plate girder theory. The finite element models are established with different aspect ratios and slenderness ratios, and the nonlinear numerical analysis is conducted in order to study the influence of VBE stiffness on the mechanical behavior of CFST frame-thin SPSWs, the deformation of VBE and the failure mechanism. The analysis results show that the CFST vertical boundary elements which are designed according to the proposed formula can fully develop the structural behavior of the SPSWs, and achieve the expected failure mechanism.