Abstract: External restraining members in Assembled Buckling-Restrained Braces (ABRBs) are mainly connected by high-strength bolts. A reduction in the restraining stiffness of the restraining members should be considered due to the sparsity and randomness of the layout of the bolts. In addition, the layout intensifies local stresses of the restraining members between bolts. In this study, the design theory of traditional pinned buckling-restrained braces was first introduced, followed by the solution procedure of the reduction coefficient of the restraining stiffness of double rectangular tube ABRBs with extended cores. Subsequently, a design method applicable for ABRBs with double rectangular tubes was proposed, considering local impact of the contact force when the cores were subjected to multi-wave deformation. Finally, two examples were provided demonstrating the validity of the proposed design method. This design method also considered a series of parameters including the gap between the core and external members, pinned connector length, extended core length, length, strength, stiffness and imperfection of the external restraining members, bolt dimensions and layout, etc.