Abstract: The immersion test of 316LN stainless steel in deoxygenated high temperature water was conducted in a titanium alloy high-pressure reactor, and the characteristics of the oxide film formed on the surface of 316LN stainless steel after immersion test were observed by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy testing. The results show that the outer oxide layer was rich in iron, contained nickel, and had low chromium. The inner oxide film was rich in chromium, iron, and nickel, manifested as two segmented regions. The concentration gradient of the main alloying elements and oxygen was large in the transition zone near the metal matrix. In the quasi uniform region between the transition zone and the outer oxide film, the concentration gradient of elements was relatively small, manifested as a gradual increase in chromium content and a gradual decreased in nickel and iron content. There was a steep change in the main alloying elements and oxygen content between the inner oxide film and the outer oxide film.