The work aims to explore effects of post-mixed water jet shot peening process on surface properties of 18CrNiMo7-6 carburizing steel. Changes in surface morphology, surface roughness, residual stress and microhardness of the specimen before and after post-mixed water jet shot peening as layer depth varied were analyzed with 3D microscope system, 3D surface morphology measuring system, X-ray residual stress analyzer and HV-1000 micro-hardness meter. During the post-mixed water jet shot peening, projectile and water would have erosion, wear and shearing effects on specimen surface, produces new pits on the surface. The surface roughness Ra increased as injection pressure P and injection target distance H increased, and it decreased as nozzle velocity v increased. The maximum microhardness appeared on the specimen surface, and the microhardness decreased as layer depth increased. When the injection pressure P was 300 MPa, the surface microhardness reached 62.8HRC, 7.35% higher than initial surface hardness value of the specimen. Residual compressive stress had a maximum inherent value σmirs. When the introduced residual compressive stress was below σmirs, the maximum residual compressive stress σmcrs increased as the injection pressure P increased, but changed slightly as jet standoff distance H and nozzle movement speed v varied. However, when the introduced residual compressive stress reached σmirs, the produced maximum residual compressive stress σmcrs equaled σmirs, but distance zm between the maximum residual compressive stress and the specimen surface still increased as the injection pressure P increased. After post-mixed water jet shot peening, specimen surface roughness changes greatly, microhardness increases to a certain degree, and the distribution of residual stresses is mainly related to the injection pressure P instead of jet standoff distance H and nozzle movement speed v.