Aimed at the clearance leakage of a constant pressure valve in polluted environment used in the integrated transmission of an armored vehicle,a twodimension geometric model of different clearance hydraulic spool valve was established.Numerical simulation for the flow field of hydraulic oils with particles in different volume fraction was performed by using Fluent software,and the flow field characteristics and changing rule of leakage were obtained.The results show that when the hydraulic oil flows into the valve port,the velocity of the oil is increased,the oil pressure is decreased,and jet flow at the valve orifice is generated.The volume fraction of particle is higher in the valve port flow beam and vortex edges but lower in the central of vortex,and it is higher in the central area of clearance than near the wall.The leakage of the hydraulic oil with polluted particles is lower than that without polluted particles.The leakage is proportional to volume fraction of particles at the same clearance,and it is reduced with the volume fraction of particles increasing.The rate of leakage reduction with volume fraction of particles increasing is generally proportional to the third power of the clearance.