Steel slag and CO2 are the two major wastes emitted by the iron and steel industry. Indirect carbonation of steel slag can achieve the dual benefits of Ca circulation and carbon emission reduction. Effective leaching of Ca from steel slag is a key step in the indirect carbonation process. However, Ca in steel slag exists in different Ca-based phases, and different active phases lead to different leaching efficiency. In order to investigate the dissolution law and reaction mechanism of the Ca-based phase of steel slag in the leaching process and the influence of the leaching process on leaching efficiency, ammonium chloride solution was employed as an extractant to leach steel slag. Furthermore, a thermodynamic calculation method was utilised to predict the dissolution behaviour of the calcium-containing phase of steel slag. The relationship between solid-liquid ratio, extractant concentration and leaching temperature on leaching efficiency was investigated by means of an orthogonal test. In addition, the mineral composition, surface morphology, element distribution and particle size change of steel slag before and after leaching were characterised by XRD, SEM-EDS, and particle size distribution. The results showed that the factors affecting the leaching efficiency of Ca in steel slag were solid-liquid ratio, leaching agent concentration and leaching temperature according to their influence. Under the conditions of solid-liquid ratio of 1:40, leaching temperature of 80℃, and leaching agent concentration of 3 mol/L, the concentration of Ca2+ in the solution was 4689.4 mg/L after leaching for 1 h, and the leaching rate was 63.1%. The results of thermodynamic calculations, XRD, and SEM-EDS analysis indicated that ammonium chloride selectively extracted calcium from the calcium-containing phase, while the iron remained in the slag to form enrichment. In this process, the particle size of the steel slag gradually decreased, and cracks and holes were generated on the surface, which was beneficial to increase the contact area between the steel slag and the ammonium chloride solution.