In order to study the wear mechanism of the wet DCT steel disc of a certain vehicle under a given working condition, a road test was carried out on a slope of 2 km in length and 30% in slope. The most severely wear steel disc was selected. The appearances of different areas and sections of the sliding friction surface were observed with a scanning electron microscope (SEM). The evolution of the section composition was detected by using energy dispersive X-ray spectrometer (EDS). Both can characterize the micro-morphological characteristics of the wear surface and the section and the oxide spalling phenomenon due to frictional heat and plastic deformation. The steel disc wears uniformly in the circumferential direction, and there are scratches and furrows of different depths in different areas along the radial direction. The contact area of the friction pair micro-protrusion body tears at high temperature. The middle diameter area which is less stressed generates a thin oxide layer of 1 μm, and the thickness of the oxide layer in the area with greater stress is about 8.5 μm. The oxide layer cracks and expands under high temperature and high pressure, and the oxide peels off to form pits of varying degrees. During the entire wear process, the main wear mechanisms of the steel disc are adhesive wear, abrasive wear, oxidative wear and fatigue wear. The greater the temperature and stress on the surface of the steel disc, the higher the degree of wear; the closer to the wear surface, the more obvious the degree of oxidation. At the beginning of sliding friction, the oxide layer is thinner, and the wear surface is slightly oxidized. The main wear mechanism is adhesive wear with slight abrasive wear; in the later stage of sliding friction, a large amount of friction heat generated increases oxides, and the main wear mechanism are oxidative wear and fatigue damage.