The work aims to provide an experimental basis for tribological application of polyimide composites in seawater. Tribological properties of friction pair consisting of 15% graphite-reinforced polyimide composite and 17-4PH stainless steel in seawater were studied using SST-ST pin-on-disk friction tester, the tribological properties under dry friction and pure water lubrication were compared. The polyimide composites had the highest friction coefficient and wear volume (0.134 and 1.930 mm3, respectively) under dry friction. Material plastic flow and adhesive peeling were present around deep furrows on the worn surface of PI composites under dry friction, polyimide composites transferred on the surface of mating plate. The wear mechanism was mainly abrasive wear, plastic deformation, adhesion and peeling under dry friction condition. Provided with pure water lubrication, the surface was subject to common material adhesion and tearing, shallow and wide furrows on the worn surface of PI composites. The wear mechanism was mainly adhesive and abrasive wear. Provided with seawater lubrication, friction coefficient and wear volume were the minimum (0.086 and 1.235 mm3, respectively). The worn surface was very smooth and only a few slight furrows were present in direction of motion, the wear mechanism was mainly abrasive wear. Graphite reinforced polyimide composites have better tribological properties in seawater than those under dry friction and pure water lubrication.