In order to improve the cleanliness of 304 stainless steel, the evolutions of total oxygen (TO), nitrogen, the composition, number density, average diameter and area fraction of inclusions were analyzed during the continuous casting tundish process. The compositions of inclusions at the end of LF was mainly CaOSiO2Al2O3, Influenced by the reoxidation of molten steel in the tundish at the casting start, a large number of small MnOrich inclusions were formed. The composition of inclusions changed to Al2O3SiO2MnO at the casting start. Meanwhile, area fraction and number density of inclusions increased, and average diameter decreased. With the casting of the molten steel in tundish, the number density and area fraction of inclusions decreased, which was related to the transition of inclusion composition from SiO2Al2O3MnO to CaOSiO2Al2O3.The thermodynamic calculation showed that the MnO content in inclusions increased with the continuous reoxidation of the molten steel by air absorption.