Abstract: 20CrMo alloy steel is commonly used to produce high-pressure pipes, gears, automobile parts, etc., and there are stringent requirements for its yield strength, tensile strength, and impact energy. In the actual production process, the existence of nonmetallic inclusions has an important impact on the properties of 20CrMo steel; therefore, studying the evolution of inclusions in the process is necessary. To further examine the evolutionary mechanism of inclusions in the overall production process, the evolution of nonmetallic inclusions in a 20CrMo alloy steel produced via the route of “Basic oxygen furnace (BOF)→Ladle furnace refining (LF)→ Vacuum cycle degassing process (RH)→ calcium treatment→ Continuous casting (CC)→ hot rolling” was studied. This process ensured a smooth production process and improved the mechanical properties of the products. Al₂O₃ was the main inclusions in the steel during LF and RH refining, which was up to 70%. After calcium treatment, CaS in inclusions increased to 59% and Al₂O₃ decreased to 21% due to the excessive mixing of calcium into the molten steel. Due to reoxidation during continuous casting, inclusions were transformed to CaO–Al₂O₃, with 50% Al₂O₃, 39% CaO, and 10% CaS. And the average diameter of inclusions also increased, which was detrimental to the mechanical properties of the steel. After cooling and solidification of the steel, CaO decreased to 5% and CaS increased to 57%. Inclusions in the steel were transformed into Al₂O₃–CaO–CaS, and a small amount of large-sized CaO–Al₂O₃ was also observed. During the rolling process of steel, the CaO content in inclusions further decreased while the CaS content and the diameter of inclusions increased. Moreover, two types of inclusions were observed in the hot-rolled plate, one being Al₂O₃–CaS compound inclusions, whose size was relatively small, and the other being CaO–Al₂O₃–CaS compound inclusions. Reasons for the formation of compound inclusions consisting of CaO–Al₂O₃ and CaS were also discussed.