Abstract: The external restraining members of assembled buckling-restrained braces(ABRB) are assembled by high strength bolts, and the layout and dimension of the bolts are closely related to how efficient they are when an external member restrains the lateral deformation of a core member. Given such facts, and based on the studies of the restraining stiffness and design method of external restraining members for a double rectangular tube ABRB, this study focuses on the internal force of the mid-span bolt considering the effects of the bolt arrangement in an external member, the core wavelength of core multi-wave deformation, and the prying force resulting from the local deformation of plates of the external member. Besides, the calculation formula of internal forces and the local strengthening recommendation of end-bolts are also put forward. As a result, the design method for the bolts applied in a double rectangular tube ABRB has been improved.