Abstract: Massive CaO(CaS)-Al₂O₃-MgO complex inclusions were found in 40Cr casting billet samples based on systematic sampling during the processes of before LF-after LF-tundish-continuous casting and electron microscopy analysis on inclusion morphology, size and composition. The formation of CaO(CaS)-Al₂O₃ complex inclusions was the-oretically calculated in the CaO-CaS-Al₂O₃ ternary phase diagram obtained by Factsage; the composition and formation process of CaO(CaS)-Al₂O₃-MgO complex inclusions were studied by further stroke treatment on surface scan distribu-tion. Factsage calculations show that the contents of components in the liquid region of the CaO-CaS-Al₂O₃ ternary phase diagram are 32% to 58% CaO, 0% to 5% CaS, and 42% to 65% Al₂O₃. The CaO content increases and the CaS content decreases gradually after calcium treatment. In combination with the scanning plane distribution of inclusions, it is found that the CaO(CaS)-Al₂O₃-MgO complex inclusions are xCaO·yAl₂O₃+mMgO·nAl₂O₃+Al₂O₃+CaS. Moreover, Ca can make Al₂O₃ become CaO-Al₂O₃ while the inclusions have very high CaS content after calcium treatment. Along with calcium treatment carrying on, CaS will gradually transform into CaO-Al₂O₃ from the inner to the outward.