The presence of reactive components, such as free lime (f-CaO) and periclase (f-MgO) in steel slag aggregate will cause serious volume instability, significantly limiting its comprehensive utilization in the building and construction materials. To address this issue, synergetic treatment of autoclaving (saturated steam pressure of 2.0 MPa) and carbonation (CO2 partial pressure of 0.1 MPa) was conducted on the hot smoldering steel slag with size of 0-4.75 mm. The effect of treatment duration on its volume stability and microstructure evolution was systematically investigated. The results showed that hydration and carbonation reaction occurred simultaneously, promoting the transformation of f-CaO, f-MgO and silicates in steel slag into more stable phases, including Ca(OH)2, Mg(OH)2, C-S-H, CaCO3, MgCO3 and SiO2 gel, thereby improving soundness. However, excessive carbonation on the particles’ surface occurred with prolonged treatment time, leading to the formation of micro-cracks appeared, which negatively affected the aggregate properties. The results demonstrated that the optimal treatment duration ranged from 2 to 4 hours. balancing the reaction efficiency and structure stability. This provides fundamental support for the future utilization of steel slag.