Magnesium slag is a solid waste produced in the process of magnesium smelting, which has low hydration activity and high carbonation activity. Through the optimization of mix proportion, magnesium slag slurry with high fluidity and high stability was successfully prepared in this paper, and a carbonized preplaced lightweight aggregate concrete was prepared by using magnesium slag slurry as grouting material. And the effect of carbonation curing on the mechanical properties and microstructure of lightweight aggregate concrete was systematically studied. The results show that the fluidity of magnesium slag slurry reaches 245 mm, which meets the technical requirements of precast concrete grouting. After drying pretreatment and 24 h carbonation curing, the compressive strength of sample reaches 24.5 MPa, and the carbon sequestration reaches 100 kg/m3. The CaCO3 crystal formed by the carbonation reaction is mainly vaterite, and a small amount is aragonite. The filling of matrix by carbonation product is the key mechanism for the strength improvement of carbonized preplaced lightweight aggregate concrete.