Abstract: Fretting wear widely exists in railway transportation. Surface treatment technology has a significant effect on enhancing the safety and reliability of locomotive operation by improving the surface wear properties of its components. A compound layer was produced on the surface of 31CrMoV9 steel sample via plasma nitriding. A tangential fretting wear test was conducted on 31CrMoV9 steel and its nitrided layer utilizing a highly accurate fretting tester in ball/flat contact mode. The fretting mechanism and damage evolution of the samples were investigated at different displacement amplitudes (D=5, 10, 50 μm) with a normal load of 20 N. In order to identify the resulting phases such as ferrite phase and combination phase, X-ray diffractometer (XRD) was conducted to examine the surface layer of distinctive samples. Morphology of the worn area was characterized by scanning electron microscopy (SEM) and white light interferometer. The chemical elements were analyzed by energy dispersive spectroscopy (EDS) and electron probe microanalyzer (EPMA). The results demonstrated that after plasma nitriding treatment, the nitrided sample developed a compound layer, diffusion layer and substrate in the direction of depth, resulting in a significant enhancement of the surface hardness. The near-surface hardness of nitrided layer was approximately 560.7 HV_0.3, which was an increase of 112.7% compared to the substrate’s near-surface hardness of approximately 263.6 HV_0.3, and the depth of nitriding influence layer was approximately 1.5 mm. As the fretting displacement amplitude gradually increased at F_n=20 N, the fretting regime of 31CrMoV9 substrate and nitrided layer changed from partial slip regime to mixed regime, slip regime, ultimately leading to an increase in the wear volume and the friction coefficient during the stabilization period. The friction coefficient of the substrate was smaller compared with the nitrided layer in the mixed regime, which was opposite to the partial slip regime and slip regime, the observed difference was attributed to the substrate sample formed a thicker debris layer than the nitrided sample in the mixed regime, which played a lubricated role. The wear mechanism of the substrate and nitrided layer were delamination, abrasive wear and oxidation wear in the mixed regime and slip regime. The compound layer generated by plasma nitriding improved the fretting wear resistance of the material, and the volume wear rate reduced by about 38.5% (D=10 μm) and 70.2% (D=50 μm). The research results provided a reference for the material selection of structural components and designing anti-fretting wear measures in the railway transportation industry and other fields.