The wet chemical reduction method was used to modify polydopamine (PDA) functionalized oxygenated carbon nitride (O-g-C3N4) nanosheets to prepare in situ growth of Cu nanoparticles (Cu NPs) decorated on nanosheets. The PDA attached to the surface of O-g-C3N4 provided a uniformly distributed platform for copper nanoparticles. It also provided rich functional groups and provided good conditions for the dispersion of Cu/PDA/O-g-C3N4 in pure oil. Through TEM testing, Cu NPs with a diameter of 5-10 nm were evenly distributed on the PDA/O-g-C3N4 nanosheets. Through the sliding test on the steel surface, it was obvious that adding 0.2 wt % Cu/PDA/O-g-C3N4 to pure oil had a lowest coefficient of friction. In addition, the wear marks on the surface of Cu/PDA/O-g-C3N4 steel discs were studied with an optical microscope and a three-dimensional profiler and its wear resistance was better than Cu NPs, O-g-C3N4 and Cu/O-g-C3N4.