Abstract: Stainless steels are widely used for corrosion resistance and as construction materials. The existence of harmful inclusions probably deteriorates corrosion resistance and easily causes nozzle clogging, surface defects, and the occurrence of cracks. Reoxidation during the casting start process significantly affects the cleanliness of molten steel, which may result in the downgrading or discarding of the steel. The production route of Al-killed stainless steel in this work is “EAF → AOD → LF → Calcium treatment → Continuous casting of round billet.” At LF departure, steel samples were taken at different moments during the casting start process to investigate the effect of reoxidation on the cleanliness of molten steel and the evolution of inclusions in the steel. It aims to achieve effective control of inclusions in the steel. The morphology, composition, amount, and size of inclusions in Al-killed stainless steel were studied using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), as well as automated SEM/EDS inclusion analysis (ASPEX). The effects of the oxygen content from reoxidation and the temperature decrease during solidification on the inclusion composition were calculated by the thermodynamic software FactSage 7.2. The evolution behavior and mechanism of inclusions during the casting start process of Al-killed stainless steel were analyzed and discussed. The findings showed that the total oxygen and nitrogen contents, as well as the number density of inclusions in the steel during the casting start process, indicated a similar change trend. They were increased to 7.4×10⁻⁵, 0.0674%, and 17.1 mm⁻², respectively, at casting 20 min, and then gradually decreased. Inclusions in the steel have been well modified by calcium treatment at LF departure, and its composition was primarily CaO−Al₂O₃−SiO₂−MgO. The effects of calcium treatment were mitigated by reoxidation during the casting start process. Inclusions in the round billet were transformed to MnO−Al₂O₃−SiO₂−CaO at casting 20 min. When the pouring time was 60 min, the cleanliness of the molten steel almost reached a steady state during continuous casting. The contents of total oxygen and nitrogen with the number density of inclusions in the steel were 3.2×10⁻⁵, 0.0628%, and 7.1 mm⁻², respectively, and inclusions were transformed back to CaO−Al₂O₃−SiO₂−MgO. Furthermore, reoxidation increases the oxygen content in molten steel and promotes the formation of MnO−Al₂O₃−SiO₂−CaO inclusions. Collision and coalescence among inclusions produce large-sized CaO−Al₂O₃−SiO₂−MnO−(MgO) inclusions in the steel. The decrease of the temperature during solidification promotes the precipitation of the MgO·Al₂O₃ spinel phase and CaO·2MgO·8Al₂O₃ phase. As a result, the Al₂O₃ content in inclusions increases.