Abstract: Taking 304 L stainless steel, the key material of the primary loop system of pressurized water reactor (PWR) as the research object, the oxide film was prepared on surface of the material by using the high temperature and high pressure circulation loop to simulate the on-site process. On this basis, the acid shutdown process at two temperatures was carried out, and the effects of acid shutdown temperature on the morphology, element composition and phase structure of the oxide film on the material surface were compared and analyzed. The results show that after pretreatment for 1 000 h, a double-layer oxide film structure was formed on the surface of 304L stainless steel. The outer oxide film was Ni-Fe spinel Ni_xCr_yFe_3-x-yO₄, and the inner oxide film with thickness less than 0.1 μm was Cr-rich spinel Ni_xFe_yCr_3-x-yO₄. After 480 h of acidic shutdown treatment at 170 ℃, the inner and outer oxide films of 304 L stainless steel were significantly dissolved, and the dissolution of the inner oxide film led to the blurring of the interface between the oxidation film and the metal matrix. After 480 h of acid shutdown treatment at 60 ℃, the spinel oxides on surface of 304 L stainless steel did not dissolve significantly, and a small amount of large granular spinel oxides were distributed on the surface, and the interface between the oxidation film and the metal matrix was clear.