Abstract: In order to study the effect of prefabricated oxide film thicknesses on the corrosion behavior of 316L stainless steel in high-temperature and high-pressure pressurized water reactor (PWR) primary water, corrosion tests were carried out to 316L stainless steels with prefabricated oxide films on surfaces in simulated corrosion environment, and the surface morphology, chemical composition and phases were analyzed by scanning electron spectroscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results show that when the thickness of the prefabricated oxide film was less than 4.46 μm, the oxide film was dense (Fe,Cr)₂O₃, which inhibited the subsequent corrosion process. With the increase of the prefabricated oxide film thickness, the (Fe,Cr)₂O₃ grains were gradually replaced by (Fe,Cr)₃O₄ grains, and the oxide film was porous and the porosity increased, so the corrosion resistance decreased gradually. When the thickness of the prefabricated oxide film was greater than 8.61 μm, the mass of the specimen after corrosion changed from increasing to decreasing, and the outer layer of the oxide film was transformed from Fe₂O₃ to Fe₃O₄, and the inner layer was FeCr₂O₄. When the thickness of the prefabricated oxide film was greater than 18.45 μm, there were obvious cracks between the oxide film and the substrate, and the oxide film was easy to fall off. There was a critical value of the thickness of the prefabricated oxide film, making a dynamic balance between the dissolution of old film and the formation of new film.