Abstract: The external restraining members of an assembled buckling-restrained brace are held together by high-strength bolts, which should be considered in the calculation of external member restraining stiffness, as the discrete layout of the connection bolts has adverse impact on the integral restraining stiffness of the external restraining members. This study puts forward the calculating method for external restraining stiffness of double rectangular tube assembled BRB. In the theoretical derivation, the external member is assumed as a vierendeel truss to consider the discrete bolt connection and then the “continuous link method” is applied to consider the bolt connection continuously, so as to quantify the adverse effect due to discrete bolts. By investigating the deformation character of the vierendeel truss, the stiffness reduction of the external restraining member is obtained, and the link stiffness between external members is finally reached by analyzing the mechanical mechanism of external members and by fitting numerical results, thus the stiffness calculation theory of external restraining members is refined. The research results are verified by the numerical analysis.