Buckling-restrained brace using steel mortar planks (BRBSM) designed to reduce plastic strain in the beam-to-column connections are being widely used in practical structures. Although a performance evaluation formula has been proposed to guide the design of BRBSM, the safety for this standard has not been assessed. In this paper, for reasonable design of BRBSM under reliability-based requirement, moment method is adopted to evaluate the relationship between the restraining index and its corresponding exceeding probability of cumulative plastic strain energy ratio based on a total of 101 previous experimental data. It can be observed that the exceeding probability of the cumulative plastic strain energy ratio changes with the increasing restraining index. To clearly indicate the reliability level of the performance of BRBSM, two boundaries with the nearly same exceeding probability of 5% and 10% respectively are then presented.