As an important raw material of magnesia based refractories, the dissolution of the fused magnesia in the CaO-Al2O3-SiO2 slags for refining a low density and high strength steel is an important factor affecting the service life of furnace lining and the quality of steel. In this paper, the dissolution behavior of fused magnesia in the slag at 1 600 ℃ was investigated by a high-temperature laser confocal microscopy system. The effects of the shape and mass of magnesia particles and the mass fraction ratio of CaO to SiO2(C/S) on the dissolution behavior were investigated. The results show that the shape of fused magnesia particles has little effect on its dissolution rate. The dissolution rate decreases linearly with the increase of mass, and it decreases firstly and then increases with the increase of C/S ratio. The dissolution curve of magnesia particles is affected by the mass, and the n values of the dissolution model can correspond to 1/2, 2/3 and 3/5, respectively. This work provides a guidance for understanding the slag corrosion behavior of magnesia based refractories, improving the quality and optimizing the performance of magnesia based refractories.