Taking the secondary O-ring of the hydrodynamic mechanical seal for nuclear main pumps as the research object,its tribological properties under dry friction and grease lubrication was tested by the UMT-3 tester.The relationship between the slip speed and the friction coefficient at different lubrication conditions was obtained.The lubrication effect of the grease was evaluated.The results show that when the reciprocating displacement is fixed,the reciprocating frequency has little effect on the friction coefficient under dry friction conditions,when the reciprocating frequency is fixed,the friction coefficient increases with the increasing of reciprocating displacement.When the reciprocating displacement is small,the grease has little effect on friction reducing,and when the reciprocating displacement is increased,the lubrication effect of the grease is enhanced.In order to improve the service life of the Oring and reduce the wear of guide ring,the axial vibration of the pump shaft should be reduced as much as possible,so that the contact interface of the seal ring is in a sticky state.