To explore the mechanical properties and microscopic mechanisms of particle system densification process，a measurement ball generation method with automatic position adjustment is proposed and a uniaxial compression discrete element numerical model of particle system is established.The evolution laws of displacement field and velocity field of particle system during densification process are analyzed，and the influence laws of particle size and loading speed on porosity and coordination number of particle system are studied.The evolution characteristics of force chain network of particle system during densification process are analyzed，and the influence laws and microscopic mechanisms of mechanical properties of particle system during densification process are revealed.The research results indicate that the densification process of particle systems can be divided into three stages：rapid densification，slow densification，and steady densification.A wider particle size distribution is conducive to improving the filling density of particles and accelerating the densification process.During the densification process，the displacement field and velocity field exhibit a centralization phenomena.The loading speed has a significant impact on porosity The porosity decreases from 35.23% to 27.50%，26.2%，24.4%，and 22.3%，respectively，at speeds of 0.05m/s，0.5m/s，1.5m/s，and 3m/s.Increasing the loading speed is beneficial for the compaction of the particle system.The larger the particle size is，the greater the particle deformation and contact strength are during the loading process.The combination of 0.8-1.5mm mixed particle sizes significantly affects the formation of force chains and the uniformity of stress，leading to a stable state of short chain dominance and long chain shrinkage during the densification process of particle system.