The effect of calcium treatment on the morphology of sulfide in 303 t 16Mn tube sheet ingots was studied by industrial experiments. The defects in forgings without calcium treatment were mainly distributed axially near the top of the ingot and radially within 1/2 of the ingot center. The defects were mainly caused by large-size type II MnS inclusions. The three-dimensional distribution of MnS inclusions in the radial center of the ingot top was detected by Micro-CT, MnS inclusions were mainly long strip and flake shape. The number density of MnS inclusions was 0.77 #/mm3, the average sphericity was 0.53 and the maximum diameter was 1370 μm. Calcium treatment of molten steel was taken after the vacuum-breaking in VD refining in industrial experiment. Thermodynamic calculation was carried out by FactSage 7.1 software, calcium treatment not only modified the original solid Al2O3 inclusions into liquid calcium aluminate inclusions, but also reduced the precipitation temperature of MnS inclusions from 1244.7 oC to 1227.9 oC. During the solidification and cooling process of ingot, Ca preferentially combined with S to form CaS, which reduced the precipitation of MnS inclusions. Calcium modified the oxide inclusions in steel before modifying sulfides, so the effective Ca/S ratio (Ca/Seff.) was proposed to characterize the effect of calcium treatment on the morphology of sulfides. Ca/Seff. was the atomic concentration ratio of effective calcium for modified sulfide to sulfur in steel, which can be directly calculated by the total oxygen content, the total sulfur content and the total calcium content in steel. This formula was suitable for low sulfur steel with high cleanliness, and the sulfur content in molten steel was less than 30 ppm. With the increase of Ca/Seff. in steel, the aspect ratio of sulfide at the top of ingot decreased gradually. When Ca/Seff. in steel was 0.22, the sulfide in steel was homogeneous MnS inclusions and the average aspect ratio was 1.59. The Ca/Seff. in the steel after calcium treatment should be greater than 0.8, and the average aspect ratio of sulfide at the top of the ingot can be controlled to less than 1.2. CaS and MnS sulfides were precipitated with calcium aluminate inclusions as the core, and the sulfides were spherical and the size was significantly reduced. As a result, calcium treatment modified the sulfide in the ingot effectively.