The influence of factors such as constraint ratio, gap-thickness ratio, and diameter-thickness ratio on the ultimate bearing capacity of end-enhanced double-steel tube buckling restrained brace (BRB) is studied using ANSYS software to simulate monotonic loading. Through the secondary development of APDL, the critical constraint ratio value range, critical gap-thickness ratio, and critical diameter-thickness ratio are analyzed. The results show that the constraint ratio is an important factor affecting the stability of BRBs and should not be less than 1.5 without considering initial imperfections. When the constraint ratio meets this requirement, the upper limit of the critical gap-thickness ratio decreases with an increase in the diameter-thickness ratio of the core tube, and the change trend is close to a straight line. The critical gap-thickness ratio should not be more than 0.18. When the constraint ratio meets this requirement, the critical diameter-thickness ratio decreases with an increase in the gap-thickness ratio, and the critical diameter-thickness ratio should not be more than 29. When the diameter-thickness ratio is too large (more than 29), it is prone to high order buckling and failure, resulting in full-section yielding.