The work aims to explore effects of process parameters (water pressure, target distance, speed and feed) of water jet peening enhancing technology on performance indicators (residual stress, hardness, roughness) of 304 stainless steel surface. The residual stress, roughness and microhardness of the surface receiving water jet peening were tested with XRD stress analyzer, optical profilometer and microhardness tester, respectively. Orthogonal test results were analyzed in significance approach, and enhancing effects provided with different water jet peening process parameters were comprehensively studied based upon multi-objective optimal design. It was found that feed rate had the greatest effect on the mechanical properties, then water pressure and speed, and finally target distance. Based upon the multi-objective optimal design, the best parameter combination for 304 stainless steel enhanced by water jet peening was obtained: water pressure as 300 MPa, target distance as 15 mm, speed as 400 mm/min and feed rate as 0.125 mm. Effects of feed rate and water pressure are mainly considered to determine water jet peening process parameters while those of speed and target distance can be neglected.