This paper is to study the tribological properties of graphene/high-density polyethylene composites under the water-lubricated condition and improve the self-lubricating and wear resistance of high-density polyethylene (HDPE). Graphene nano-sheets (GNS) were used to modify high-density polyethylene material. The RTEC wear test apparatus was used to test the tribological performances of modified composites under water-lubricated conditions. The friction and wear mechanism of graphene/HDPE under water lubrication condition was revealed by analyzing the typical mechanical properties, coefficient of friction (COF), wear morphology, and element composition and distribution of the contact surface of the friction pairs. The results showed that the tensile strength, tear strength and Shore hardness of the modified composites increased first and then decreased with the increasing of GNS mass content, and HDPE modified by 1.5% GNS exhibited the highest values of 19.81 MPa, 31.34 MPa, and 92.6HSA, respectively. The average COF and volume stroke wear rate of the modified composites decreased with the increasing of GNS mass content, and the average COF and wear rate of HDPE modified with 1.5% GNS were reduced by 53.6% and 73.9% respectively compared with pure HDPE. The carbon element mass fraction in the wear area of the Si3N4 ceramic ball sliding against with HDPE modified by 1.5% and 0.6% GNS for 3600 s wear test were approximately 3.5% and 0.3% respectively, which indicated the higher mass content of GNS was beneficial to the formation of the GNS lubricating layer on the micro-interface, and resulted in reducing the COF. In a summary, GNS significantly affected the self-lubricating performance and wear resistance of HDPE. Appropriate mass content of GNS promoted the formation of a GNS lubricating layer at the wear interface of HDPE, and reduced the COFs and wear volumes. The results in this study contribute to design the water-lubricated stern tube bearing polymers with low friction and high wear resistance.