Abstract: Compared with traditional bucking-restrained brace (BRB), the assembled buckling-restrained brace (ABRB) possesses a more complicated design theory due to its special composition. Based on the understanding of the interaction between the core and the outer restraining components, this paper presents a design method for the stability verification for a four-angle ABRB. Firstly, by using a continuum method, a simplified method to determine the restrain stiffness of the outer restraining components is given. Then with the assumption that only one-flexural-direction displacement is produced, the limit value of the restrain ratio of a four-angle ABRB is derived based on the understanding to the stress state on the outer restraining angle steels. Based on the above conclusions, a method to determine the restrain ratio is further improved by taking the stresses brought by the flexural displacement in the second direction into account, which can assure both the overall and local stability of the ABRB.