Based on the theory of friction and lubrication, the models of the rotary seal under the condition of fluid lubrication and boundary lubrication were established respectively. Relations between force characteristic and structural parameters of the sealing ring were analyzed. Lubrication state between the sealing ring and the surface of the shaft groove was also discussed. The expression of calculating friction torque under the condition of boundary lubrication was deduced. Experiments have been done on the rotary seal test bench designed specially. Testing results validated the calculation formula.