Abstract: PH13-8Mo is a precipitation-strengthened, martensitic stainless steel with ultra-high strength, and satisfactory toughness and plasticity. It is generally utilized in the fields of aviation and traditional energy because of its remarkable mechanical properties and corrosion resistance, as well as its stable performance in harsh service environments. Because of the wide applications of PH13-8Mo stainless steel and the complex corrosive environments it faces, its corrosion resistance is of great significance for deciding the lifetime and safety of aircrafts and ships. However, limited by factors, including a long outdoor exposure test cycle and a large professional experimental site required, only a few reports exist on the atmospheric corrosion behavior and mechanism of PH13-8Mo stainless steel, especially the influence of chemical pre-passivation on the steel still remains relatively uninvestigated. Therefore, the outdoor exposure tests of two samples of PH13-8Mo stainless steel, with and without nitric-acid-passivated film, respectively, were performed in a semi-rural atmospheric environment in Beijing for five years. The effect of the pre-passivation treatment on the corrosion behavior and mechanism of PH13-8Mo stainless steel was investigated by observing the surface morphology, using the mass loss method, analyzing the passivated film and corrosion products, testing the mechanical properties, and conducting fracture analyses. The results show that the pre-passivation treatment with nitric acid reduces the pitting corrosion and decreases the corrosion rate. The pre-passivation treatment with nitric acid delays the destruction of Cl⁻ on the passivated film and also delays the nucleation of the pitting by increasing the hydroxide content and the atomic ratio of Cr/Fe of the passivated film, and it increases the surface Kelvin potential as well, further enhancing the protectiveness of the surface film. Additionally, the pre-passivation treatment with nitric acid reduces the loss in the mechanical properties after long-term exposure to the semi-rural atmospheric environment, although it has little effect on the fracture mode, and both the steel samples exhibit the typical morphologies of a ductile fracture.