The dynamic water loop system was utilized to simulate the primary water environment of a Pressurizdemonstrate thated Water Reactor (PWR), and the effect of aluminum on the film-forming behavior of 304L stainless steel in the primary water environment was investigated. The experimental results after oxidation in a lithium hydroxide solution for 300 h, the surface of 304L stainless steel turned black, and the size of oxide particles on the microscopic surface was approximately 500 nm. After oxidation in a lithium hydroxide solution containing aluminum for 120 h, the surface became pale yellow, and there were 150-nm oxide particles on the microscopic surface. Subsequently, when it continued to be oxidized in the lithium hydroxide solution for another 300 h, the surface appeared dark red, the size of the oxide particles on the microscopic surface was about 200 nm, and the oxygen content in the oxide film increased. The results of X-ray photoelectron spectroscopy analysis showed that the aluminum peak on the surface of 304L stainless steel increased after the aluminum-alkali passivation process. However, due to the fact that the atomic radius of aluminum is larger than that of nickel and iron, aluminum atoms have difficulty in aluminum atoms to enter the crystal structure of AB2O4-type oxides. The growth rates of the oxide films on 304L stainless steel and 690TT (an alloy material) were slowed down, possibly due to [Al(OH)4]− was attached to the metal surface.