Soft soil is a kind of widely-distributed soil with poor engineering properties, and its stabilization using cement does not meet the requirements of modern social development. The composite binder of carbide slag-reactive MgO and CO2 were used to carbonize and stabilize silty soil, and two variables of the binder dosage (single carbide slag) and the carbide slag-MgO ratio were set. The engineering properties of the carbonated/stabilized soil were analyzed from the physical mechanics and chemistry aspects. The results showed that: ① when the content of only carbide slag was 20%, the soil strength was the highest after carbonation. With the addition of a mixing binder, the strength of the carbonated sample increased first and then decreased with increasing the incorporation ratio of carbide slag.② The density of the carbonated samples increased significantly compared with that before carbonation. When calcium carbide slag was only added, the density of the carbonated samples increased with the increasing content of carbide slag. With the addition of a mixed binder, the variation in the density of the carbonated samples was consistent with the strength. ③The pH of the carbonated samples decreased significantly compared with that before carbonation. With the addition of carbide slag, the pH increased with the increase of carbide slag content after carbonation. With the addition of a mixing binder, the pH decreased obviously after carbonization when the carbide slag-MgO ratio was 4:3. ④ the proportion of coarse particles (> 75 μm) in the carbonated samples significantly increases, and it decreased with increasing the content of carbide slag. However, there is no significant correlation between the proportion of coarse particles and the carbide slag-MgO ratio when the mixing binder is added. The results would be beneficial to improving the utilization of solid waste resources and expanding the engineering application of carbide slag–MgO carbonation technology in the treatment of soft soils.