Lime is one of the slag making materials in the steelmaking process. Its rapid and sufficient dissolution can accelerate the dephosphorization and desulfurization reaction rates. The dissolution process of lime in CaO-Al2O3-SiO2(-MgO) slag system at 1550℃ was studied using the single hot thermocouple technique (SHTT). The results showed that when lime dissolved in CaO-Al2O3-SiO2 slag without MgO, its apparent radius ratio gradually decreased with increasing dissolution time, and the dissolution rate remained relatively stable throughout the entire dissolution process, with the shortest dissolution time among all systems. When lime dissolved in CaO-Al2O3-SiO2 slag containing MgO, the dissolution time increased gradually with increasing MgO content. According to the change in dissolution rate, the dissolution process could be divided into three stages. In the first and third stages, the lime dissolved quickly. However, in the second stage, the formation of MgAl2O4 solid phase product layer hindered the dissolution of lime. The analysis of lime dissolution kinetics showed that in CaO-Al2O3-SiO2 slag without MgO, the dissolution was controlled by boundary layer diffusion as the limiting step. In contrast, in CaO-Al2O3-SiO2 slag with MgO, the dissolution limiting step varied with both dissolution time and MgO content. Specifically, for Slag 2 (3wt% MgO) and Slag 3 (6wt% MgO), the dissolution limiting step was controlled by interfacial chemical reactions in the early stage, but shifted to boundary layer diffusion control in the middle and late stages. For Slag 4 (9wt% MgO), the limiting step was interfacial chemical reaction only in the initial part of the first stage, with product layer diffusion becoming the controlling factor thereafter.