Leakage prevention and plugging of drilling fluid is a key technical measure to ensure drilling safety， protect reservoirs， reduce economic losses， improve drilling efficiency and environmental protection. Due to the limitations of actual observation and experimental conditions in the field， it is difficult to explore the plugging mechanism of leaky formations. In order to better explain the particle plugging mechanism of fractured leaky formations， this study used COMSOL Multiphysics to establish a plugging model for fractured formations， and analysed the effects of particle concentration， particle size and particle gradation on the plugging efficiency of fractured leaky formations， so as to reveal the specific mechanism of particle plugging. The results show that： （1） The plugging efficiency is positively correlated with the particle size. The larger the particle size， the shorter the plugging time under the same concentration. （2） There is an effective blocking range for particle concentration， and effective blocking cannot be formed when the particle concentration is lower than the lower limit of the range， while the increase in concentration has little effect on the blocking efficiency when the particle concentration is higher than the upper limit of the range. （3） The particle gradation can synergistically improve the blocking efficiency and blocking densification. When the concentration of large-sized particles is the same and the mass fraction of small-sized particles concentration is increased by 1%， the final leakage velocity is reduced by 35%； When the concentration of small-sized particles is the same and the mass fraction of large-sized particles concentration is increased by 4%， the formation time of crack bridge plugging is reduced by 50%. The small particles in the plugging particles mainly affect the densification of the plugging layer， while the large particles mainly affect the plugging efficiency.