Dissolution kinetics of CaO·2Al2O3 (CA2) particles in a synthetic CaO–Al2O3–SiO2 steelmaking slag system have been investigated using the high-temperature confocal laser scanning microscope. Effects of temperature (i.e., 1500, 1550, and 1600 °C) and slag composition on the dissolution time of CA2 particles are investigated, along with the time dependency of the projection area of the particle during the dissolution process. It is found that the dissolution rate was enhanced by either an increase in temperature or a decrease in slag viscosity. Moreover, a higher ratio of CaO/Al2O3 (C/A) leads to an increased dissolution rate of CA2 particle at 1600 °C. Thermodynamic calculations suggested the dissolution product, i.e., melilite, formed on the surface of the CA2 particle during dissolution in slag with a C/A ratio of 3.8 at 1550 °C. Scanning electron microscopy equipped with energy dispersive X-ray spectrometry analysis of as-quenched samples confirmed the dissolution path of CA2 particles in slags with C/A ratios of 1.8 and 3.8 as well as the melilite formed on the surface of CA2 particle. The formation of this layer during the dissolution process was identified as a hindrance, impeding the dissolution of CA2 particle. A valuable reference for designing or/and choosing the composition of top slag for clean steel production is provided, especially using calcium treatment during the secondary refining process.