Water toughened high manganese steel is treated by laser shocking, and the micro-structure is analyzed by H-800 transmission electron microscope and JSM-5610LV scanning electron microscope. The results show that the hardness of high manganese steel will increase significantly after laser shock processing (LSP) and hardening will occur. The hardness of the impacted center increases from 219 HV to 486 HV, which rises about 122%. In the LSP, a more severe plastic deformation is undergone on the materials, a large number of dislocations are generated, and thus the dislocation block and the cellular structure of dislocations form. With the deformation increasing, a large number of twins are generated in the structure. So the grain boundaries, phase boundaries and other interfaces are destroyed by twinning and dislocation interaction, and grain refinement forms. Dislocations and twins share the role with the fine grain strengthening, so a substantial increase of the performance is emerged in high manganese steel.