This paper introduces the structural design and operation principle of rotary hydraulic cylinder. Seven spiral flows with different clearance were designed. The models were built by Pro/E. Structure meshes were generated by using Gambit. Based on laminar model and SIMPLE algorithm, the interior flow field in different clearance and the same clearance at different rotation rate were numerically simulated and analyzed with Fluent. The relationship of clearance, rotation rate and viscous force, maximum temperature, stiffness, flow rate were obtained. Simulation results show that clearance’s influence on internal leakage is greater than rotation rate, at the clearance of 0.05mm～0.10mm is relatively suitable.