In order to reveal the mechanism of the high-frequency vibratory stress relief, the effect of the evolution of the dislocation density with the help of high-frequency vibration on the residual stress relaxation. The Williamson-Hall (WH) method and the hole-drilling method were used to evaluate the dislocation densities and the residual stresses in the Cr12MoV steel quenched specimen before and after high-frequency vibration. Based on the experimental results, the multi-freedom system of the crystal grain was developed based on the microscopic dynamics theory in order to analyze the mechanism of high-frequency vibration activated dislocation motion. In addition, the governing equation of the dislocation density evolution with the aid of high-frequency vibration was proposed in order to reveal the microcosmic mechanism of the high-frequency vibratory stress relief. The results show that the dislocations inside the materials were activated with the help of high-frequency vibration. The dislocation density evolution process mainly consists of the dislocation accumulation process and the dislocation annihilation process. And the dislocation annihilation process has a dominant process with the help of high-frequency vibration, which results in that the dislocation density inside the materials can be reduced. The degree of the lattice distortion can be reduced due to the decrease of the dislocation density. As a result, the residual stress inside the materials can be relieved by the high-frequency vibratory stress relief.