In order to obtain the lubricating capabilityof screw rotary cylinder, its structural design and operation principle were introduced. Seven screw pairs with different radial clearance were designed. The models were built by Pro/E. Structure mesh was generated by using Gambit. Based on laminar flow model and SIMPLE algorithm, the interior flow field in different radial clearances and the same radial clearance at different inlet pressure were numerically simulated and analyzed with Fluent. The relationship of loading force, stiffness, maximum temperature, flow rate and radial clearance were obtained. Simulation results show that the performance of oil lubricated screw pair is the best at the radial clearance of 0.10 mm. At the same radial clearance, when back pressure is constant, with inlet pressure increasing, loading force, stiffness, flow rate and maximum temperature increase completely.