Abstract: Oil sleeve is an important sealing element in hydraulic drill rig, whose sealing performance immediately affects the volumetric efficiency of hydraulic system. Through calculating the sealing clearance variation resulted from the deformation of oil sleeve and the main shaft under action of pressure and heat, simulating and analyzing the internal flow field of oil sleeve with CFD technology, we got the pressure distribution inside the sealing clearance and the relationship between leakage rate and pressure or temperature. The results have great reference value for the design of working pressure and determination of the operating temperature range.