The objective of this paper is to study on the effect of the relief groove angle expressing the position in reducing noise of a swash plate axial piston pump. The relief groove is an important structure for reducing noise. Unfortunately the effect of the position of the relief groove is quite seldom explained in research reports. This paper focuses to this problem. A detail simulation model including the structure and position of the relief groove as well as the compressibility of oil film is built. The optimal shape of the relief groove is obtained by analyzing the flow ripple and cylinder pressure of the piston pump with the different shape of the relief groove on the port plate. The effect of the relief groove position and different structures on noise reducing is found by analyzing the flow ripple, cylinder pressure, sizes of relief groove and its location in different positions on the port plate. In this way the influences of the relief groove on reducing noise are found and the optimization of relief groove is obtained.