Solidliquid twophase particles turbulence in clearance seals of slide valves was numercially analyzed with using of Euler multiphase flow model, extended kε turbulence model and SIMPLE algorithm based on Fluent software. The effects of pollution particles with different diameters and volume concentrations on friction of slide valve core were anylased. The results show that the friction increases approximately in direct proportion to the particle volume concentrations. With the increasing of the diameters, the friction increases firstly and then decreases, the particles called “sensitive particles” cause the biggest friction, they have a diameter of 0012 mm which approximately equals to the thickness of the clearance. In hydraulic system, in order to reduce valve friction and wear, improve the performance of hydraulic slide valve and improve the application performance of the whole hydraulic system, a right size of the filter or filter core is adopted to filter out the particles with sizes near “sensitive particles.”