The work aims to study on the surface of 45 steel strengthened by high pressure pure water jet, and analyze the influence of technological parameters on the strengthening surface residual compressive stress and surface roughness. The or-thogonal experimental method was used to study the influence of jet pressure, standoff distance, transverse speed, feed rate and cycle times on the surface quality. Minitab software was used to analyze the test results, study the influence law, and explore the best process combination. From the research, the order of influence of process parameters on residual compressive stress is as follows: times > feed > pressure > traverse speed > standoff distance. The order of influence on surface roughness is pressure > times > feed > traverse speed > standoff distance. As to the surface quality, the order is pressure > standoff distance > feed > times > traverse speed. The optimal process parameters are obtained: jet pressure 200 MPa, standoff distance 10 mm, feed rate 0.5 mm, cycle times 2 and transversal speed 100 mm/min. In a few words, the effect of high pressure pure water jet on the surface hardening of 45 steel is remarkable. The surface residual compressive stress is improved obviously while the surface roughness does not increase obviously, and the surface roughness is lower than 1.1 μm.