The primary magnesium production is accompanied by a large amount of magnesium slag (MS) discharge. The low hydration reactivity of γ-Ca₂SiO₄ (γ-C₂S) and MgO in MS results in the volume stability issue and low utilization rate of MS. To eliminate the issue, this study proposes to pre-autoclave the MS slurry to boost the hydration of γ-C₂S and MgO and then utilize their hydration products to prepare cementitious materials by carbonation curing. MgO from MS and prepared γ-C₂S are firstly employed as study objects respectively, for they are the main contents of magnesium slag. The results indicate that pre-autoclaving treatment can strongly elevate the hydration degree of MgO from MS, this can substantially solve the volume stability issue of MS. Meanwhile, the pre-autoclaving of γ-C₂S induces the generation of crystallized and amorphous C-S-H products, and both products could promote the carbonation reaction when compared to γ-C₂S. The carbonation degree of pre-autoclaved MS firstly increases and then decreases with the rising pre-autoclaving temperature, and the optimal pre-autoclaving temperature for MS carbonation is 160 °C, at this time, the powdered MS can be simply carbonated fully. The sample made of pre-autoclaved MS and then subjected to 4 h carbonation could achieve the compressive strength of 29 MPa. with good soundness. During volume stability testing, the volume expansion rate of a carbonated MS sample with pre-autoclaving was 0.07%, which is significantly lower than the normal requirement of 0.5%. This research offers a novel approach to utilizing magnesium slag in building materials and contributes to carbon reduction.