Abstract: Considering that the viscosity of the hydraulic oil will changeaccordingly when the temperature and pressure vary, the flow rate equation andleakage rate equation of the annular spiral flow in Hydraulic Rotary Actuatorare derived based on continuity equation, N-S equation and energy equation.Using the CFD simulation software ``Fluent'', the flow field and the leakagelaw of the internal annular spiral flow are simulated. Comparing the simulationresults with the theoretical calculation of the flowing annular spiral flow, the theoretical analysis is verified, which may provide atheoretical basis for studies of internal leakage and volume depletion in HydraulicRotary Actuator.