More than half of the defects caused by metal particle discharge in the oil chamber of high-voltage on-load tap-changer seriously affect long-term operational reliability. The movement law of metal particles in insulating oil during the disconnection process of on-load tap-changers and its impact on the dielectric properties of insulating oil are investigated, which can provide a theoretical basis for the diagnosis of insulation status of on-load tap-changers and the design of oil gap insulation margin. Firstly, theoretical analyses of fluid field, metal particle dynamics, and electric field are presented, and the coupling relationship among the three is explained. At the same time, a two-dimensional simplified simulation model of the on-load tap-changer contact is established, and the distribution of eddy currents in the fluid field, the movement law of metal particles, and the movement trajectory of charged metal particles are obtained. The simulation results show that during the on-load tap-changer movement, the particle concentration at the center of the on-load tap-changer gap is the highest, and a "small bridge" is formed. The influence of changes in metal particle concentration distribution on the dielectric properties of insulating oil is calculated using the dielectric constant and conductivity models of the mixture. A method of calculating according to concentration zones and then paralleling is proposed to improve the accuracy of dielectric parameter calculation.