The dissolution of alumina-based refractory ceramics in CaO-Al2O3-SiO2 slag melts was performed based on the in-situ observation system of an ultra-high-temperature laser confocal microscope, and the effect of the CaO/SiO2 slag mass ratio (C/S ratio) on the dissolution rate of alumina-based refractory ceramics was investigated. The results indicate that the dissolution rate increases with an increase of the C/S ratio and is mainly controlled by diffusion. During the early stage of dissolution, for all C/S ratios, the dissolution process conforms to the classical invariant interface approximation model. During the later stage of dissolution, when the C/S ratio is ≥6, the dissolution process is significantly different from the model above because of the formation of a thick interfacial layer, which can be explained by dissolution kinetics.