Abstract: The stiffness and the strength of the outer constraint component are crucial to the properties of buckling-restrained braces (BRBs). In the design, both the local buckling and the flexural buckling should be considered. All-steel BRBs are more vulnerable to the failure in local bucking mode than those restrained by steel tubes filled with concrete, because they are not constrained by concrete. This study focuses on the local buckling of the all-steel BRBs. Firstly, the contact force between the core and the outer tube was analyzed. Then, the formula of the bearing capacity of the outer tube was deduced. Finally, the concept of constraint ratio, which was used in the flexural buckling, was introduced and used in the local buckling. The expression of the constraint ratio was obtained, and its lower limit value to prevent the all-steel BRBs from local buckling was suggested. The results indicate the constraint ratio depends on the ratio of the thickness of the tube to that of the core, the ratio of the width to the thickness of the core, the ratio of the gap to the thickness of the core, and the axial strain. All of those influencing factors should be considered in the design of the all-steel BRBs.