In order to improve effect of VSR（vibration stress relief）for large machine tool body, the VSR process parameters must be optimized. This paper studies mechanism of VSR and determination principle of process parameters. Using finite elements method, the vibration modal analysis of large machine tool body are applied to determine VSR frequency, exciting position, supporting position and detecting sensor position. Based on analysis above, the harmonic response analysis is applied to work out relationship between exciting force and dynamic stress. In this way, the exciting force can be determined. The experiment result shows the determination method of VSR process parameter in this paper is feasible and effective. This method is worthy of application and dissemination for large size castings.