Abnormal wear of the cylinder liner and piston ring in a marine diesel engine will lead to the failure of the diesel engine and colossal energy loss. Using lubricating oil additive with good anti-friction and anti-wear properties effectively inhibits the abnormal wear of cylinder liner and piston rings in diesel engine. The potassium borate/graphene (PB/GN) nano-composite lubricant additive suitable for a marine diesel engine was prepared in this study. Its tribological properties in 500 N base oil were investigated using a pin-on-disk tribotester under various loads and temperature. The frictional surface was comprehensively analyzed using field emission scanning electron microscopy (FESEM), energy dispersive spectrometer (EDS), x-ray photoelectron spectroscope (XPS) and Raman to explore the lubrication mechanisms of the PB/GN nano-composite additive. The results indicated that the PB/GN nano-composite additive could substantially improve the tribological performances of 500N base oil. This could be attributed to the good thermal conductivity of graphene and the tribofilms composed of C, FeO, Fe2O3 and B2O3 formed on the worn surface. The synergy of PB and GN enhanced the tribological performances of base oil.