Abstract: Controlling the morphology of MnS inclusions is of great significance for improving the mechanical and machinability properties of sulfur-containing steel. The effect of calcium content on MnS inclusions in 42CrMoS4 sulfur-containing steel was investigated. An automatic scanning electron microscope and an energy dispersive spectrometer were used to systematically characterize the characteristics（quantity, size, morphology, distribution, etc.） of MnS in the steel. The results show that calcium content has a significant effect on the morphology control of MnS in 42CrMoS4 sulfur-containing steel. Two types of sulfides exist in the calcium-treated 42CrMoS4 steel: pure MnS and CaS-MnS. In the continuous casting billet, pure MnS has an irregular morphology, while CaS-MnS is nearly spherical. After hot rolling, pure MnS undergoes significant deformation along the rolling direction and most of it presents an elongated shape. CaS-MnS deforms slightly and remains nearly spherical. A higher calcium content can increase the proportion of spherical CaS-MnS sulfides and reduce the aspect ratio of MnS simultaneously. When the calcium content increases from 0.000 9 wt% to 0.001 5 wt%, the size of MnS decreases, and the number proportion of MnS with an aspect ratio greater than 3 reduces from 15.3% to 10.6%. Thermodynamic calculations were carried out using FactSage software to clarify the action mechanism of calcium on MnS. After calcium treatment, the Al deoxidation product Al₂O₃ inclusions transform into CaO-Al₂O₃ with a lower melting point. After the addition of S element, CaO in the CaO-Al₂O₃ oxides is thermodynamically unstable and reacts with the dissolved S in the molten steel to form CaS. During the solidification process, the CaO-Al₂O₃ oxides rich in CaS can serve as effective nucleation cores for MnS inclusions. Eventually, CaS combines with MnS to form fine and nearly spherical CaS-MnS sulfides.