The MgO expansive agents (MEA) can alleviate the detach between concrete infill and steel tubes in concrete-filled steel tube (CFST). But its restrained volume deformation and stabilizing content are still unclear, as excess MEA may cause expansion and cracking of the concrete. Thus, the effects of different active MEA on the volume stability of concrete were evaluated, and the mechanism was analyzed by means of thermogravimetric, mercury intrusion porosimetry and scanning electron microscope. The results show that the restrained expansion ratio at 270 d increases with increasing MEA content, and the higher the MEA activity, the smaller the ultimate restrained expansion ratio and the earlier the expansion time window. For R–MgO, M−MgO and S–MgO expansive agent, their expansion window in concrete is 0–28 d, 7–150 d and 28–210 d, respectively and the stabilizing content is 9–12%, 9–12% and less than 9%, respectively. An appropriate content of MEA allows the hydrated Mg(OH)2 grew in situ and adhere to the surface of MEA particles, thus refining the pore structure and providing well restrained expansion behavior and volume stability. But excessive MEA result in some Mg(OH)2 grew in the cracks and expanded the pores, thus loosens the pore structure and deteriorated the volume stability.