In steelmaking, CaO-containing inclusions exceeding 13 μm, which was named as "DS-type" inclusion in Chinese national standard GB/T-10 561-2005, should be controlled in Al-killed steel for automobile part (such as gears, bearings, and crankshaft) because it is easy to become a source of fatigue cracks in steel. [1][2][3][4][5] Monnot et al. [6] found that CaO-containing inclusion with larger size is more harmful than other type inclusions. In 2007, Kawakami et al. [7] proposed that intrinsic reaction is one of the important reasons for the formation of CaO-containing inclusion in the refining process. Miao et al. [8,9] found that spinel inclusion served as a core wrapped in some CaO-containing inclusion with large size, which indicates that these inclusions were transformed from the reaction between spinel inclusions and dissolved Ca in the molten steel during the refining process, and the researcher concluded that controlling the concentration of Ca, Al, and O in the molten steel can affect the formation of such inclusions. Some researchers of Baosteel [10][11][12] studied the formation mechanism of CaO-containing inclusion with large size and found that the concentration of Ca was an important factor for the formation of such inclusion in gear steel. They proposed that Ca treatment should be prohibited in the industrial production of gear steel. Jiang et al. [13] found that CaO-containing inclusion can collide and agglomerate at the solid-liquid interface and then form inclusions with a larger size during solidification. From the above research, it can be seen that the formation, agglomeration, and growth of endogenous CaO-containing inclusions with small size is one of the significant reasons for the formation of CaOcontaining inclusions with large size because there are sufficient kinetic conditions to promote the polymerization and growth of CaO-containing inclusions in industrial production. Therefore, it is a method to control CaO-containing inclusions with large size by preventing the increase of Ca concentration and the formation of large amounts of CaO-containing inclusions with small size.Much literature revealed the formation mechanism of CaO-containing inclusions in Al-killed steel. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] It can be summarized as 1) Ca treatment modified MgO-Al 2 O 3 and Al 2 O 3 to calcium aluminate inclusion [14][15][16][17][18][19][20][21] ; 2) High basicity slag supplied dissolved Ca to steel by slag-steel reaction and modified