In order to further expand the application field of medium manganese steel, its wear property and strengthening mechanism under the slurry erosion wear were studied in this paper. At different erosion angles and velocities, the erosion-wear resistance of medium manganese steel was superior to the martensitic wear-resistant steel. The formation of wear-hardened layer caused by the work-hardening effect was the fundamental reason for wear strengthening. The depth of the wear-hardening layer was nearly 400 µm and surface hardness was above 495 HV. The wear strengthening mechanism was attributed to the combined strengthening, including the martensitic transformation, twin, and dislocation strengthening. The maximum value of erosion wear-rate appeared at 60 deg erosion angle and the minimum value occurred at 90 deg erosion angle. The erosion-wear damage included three aspects of the micro-cutting, erosion peeling caused by the plastic deformation, and local gouging abrasion. The change of erosion angle led to the change of normal and tangential force components of erosion particles on the wear surface, which changed the main wear mechanism of erosion wear. With the change of erosion angle from low to high, the main wear mechanism gradually changed from the micro-cutting wear to the erosion peeling wear and local gouging abrasion.