Abstract: In order to reveal the mechanism of vibration compaction for high-speed railway fillers, a series of typical graded gravel fillers which belong to the surface layer of the subgrade are used to the investigation object. Firstly, an evaluation system for the compaction quality assessment indices of continuous dry density \rho _\textrd , dynamic stiffness K_\textrb and modified foundation coefficient K_\text20 is established based on the self-developed vibration compaction apparatus. Secondly, the graded gravel was scanned by X-CT during the vibratory compaction. The compaction mechanism of graded gravel was revelad based on the the dynamic evolution of coarse particles. The results show that \rho _\textrd of the graded gravel fillers exhibits a rapid rise-slow rise tendency. However, K_\textrb and K₂₀ present a rapid rise-slow downward tendency. Moreover, the compaction locking point T_\textlp is proposed to quantify the inflection point of graded gravel fillers during the vibration compaction. During the vibration compaction process, the stability of vibration compaction occurs when 60% of the coarse particles tend to be horizontally arranged in the long axis ( T_\textlp state). And then further compaction results in the occurrence of surface grinding-based crushing phenomenon of coarse particles to destroy the skeleton stability. Finally, the validity of the mechanism is verified through the discrete element simulation. The research results can reveal the vibration compaction mechanism from the fine view level and provide a new method for the intelligent rolling quality assessment of high-speed railways.