In order to improve the calculation accuracy of core steel strain, a formula for the core strain of buckling-restrained braces（BRBS） addressing the stiffness of BRB-gust plate was proposed so as to ensure BRBS yielded as designed. The stiffness of the welded joint plate is firstly derived, and the core strain formula considering the stiffness of the gust plate is subsequently established by using the geometric relationship between the frame and BRB. The pseudo-static test of the full-scale buckling-restrained brace frame and finite element analysis were conducted to verify the accuracy of the formula. The results from the formula，experiments, and finite element analysis are all in good agreement, and 60% of accuracy improvement can be achieved when calculating the story drift at the time of BRBS yielding, which demonstrates that the proposed strain formula is applicable to the optimization design for the core steel of BRBS.