Based on theoretical and experimental analysis, the mechanical characteristics of steel plate shear walls with trilateral constraint in moment-resisting frame were studied. Based on the strip model of thin plate, the proposed model of steel plate shear walls with trilateral constraint was presented. The calculation formula of the shear strength of steel plate with trilateral constraint was presented in a simplified model, and the relationship between the angle of the tension field and the constraint stiffness of angle iron was derived. The four steel plate shear walls of the specimens with various ratio of width-thickness were tested by jack. The result has indicated that the calculation formula is better for steel plate with more than 143 of ratio of width-thickness, and the precision of the calculation formula of shear strength can meet the requirement. The principal stress direction and distribution measured by experiments inosculate completely with the simplified model. And the little ratio of width- thickness of steel plate required adequate anchorage to forgo failure of boundary. The accuracy of the calculation formula of shear strength decreased with the failure between the steel plate and the boundary frame.