Water jet peening (WJP) is a surface enhancement technique that can use the impact pressure to induce compressive residual stress in the narrow concave area of metal components. For WJP on the inclined surface, this research reveals the impact pressure evolution and the forming mechanism of compressive residual stress field (CRSF). Mathematical models of predicting the critical inclined angle θc and the maximum water hammer pressure Pm are developed. Besides, a 3D dynamic finite element model of WJP is developed. Then, the simulation model is verified by the experimental results of the inclined surface. Moreover, the influence of parameters such as inclined angle θ, jet velocity v, and jet diameter d on θc, Pm, and CRSF is investigated by simulation. The results indicate that Pm essentially determines the CRSF, and WJP parameters indirectly affect the CRSF by changing Pm. v determines θc, and θc increases with increasing v. θ and v determine Pm, and Pm decreases with increasing θ while increases with increasing v. The magnitude and depth of CRSF decrease with increasing θ while increases with increasing v. But d only affects the depth of CRSF, which increases with increasing d.