The influence of different axial loads on the tribological properties of Hydrogenated Nitrile Rubber (HNBR) under oil-based mud containing hard particles is studied through experimental and theoretical approaches. Friction and wear tests were performed by using pin-on-disc friction pairs. The findings indicate that the tribological properties of HNBR vary with axial load and wear time. The total friction coefficient can be divided into an adhesion term, a hysteresis term and an abrasive term through the analysis of tribological mechanism. The hysteresis term is due to micro-strain on the HNBR surface and overall deformation of the HNBR specimen. The variation of tribological properties can be well explained through the above theory and the hysteresis term is the main influence factor.