Polytetrafluoroethylene (PTFE), which has an extremely low coefficient of friction and good thermal and chemical stability, is widely used as a self-lubricating material. In this study, the thermal oxidative-ageing mechanisms of PTFE were studied using infrared spectroscopy and molecular dynamics simulations, and the influence of the thermal oxidative-ageing time on its tribological properties was investigated under a high load (20 MPa). Results indicated that thermal oxidative-ageing resulted in oxidation fracture, generating CF2O products, followed by further oxidation to CFO2 as ageing time increased. The dynamic friction coefficient of PTFE fluctuated significantly under heavy load, and its wear rate fluctuated over a small range but showed a decreasing trend with increased ageing.