In order to investigate the friction and wear properties of drilling fluid PCD (Polycrystalline Diamond) thrust bearing under various working conditions in ultra-deep drilling, oil-based drilling fluids with varying oil-water ratios are introduced in high temperature (125 °C) friction and wear tests. The results show that: compared to oil-based drilling fluids, water-based drilling fluid fails to establish stable boundary lubrication through an oil film at the friction-sliding interface. Under high-temperature friction conditions, water-based drilling fluids are prone to vaporization, leading to severe dry friction. When the oil-water ratio of the drilling fluid is 100:0, the pure oil-based drilling fluid hinders the dissipation of friction heated generated at the sliding interface, resulting in thermal damage to the PCD compacts. In oil-based drilling fluid environments with oil-water ratio ranging from 20:80 to 80:20, the formation of a graphite tribo-chemical film and the passivation of dangling bonds on the PCD compacts surface contribute to a lower friction coefficient and wear rate. Among these oil-water ratios, the PDC compacts tested in the 80:20 oil-water ratio exhibited the optimal tribological properties, which can be attributed to the synergistic effect of a smooth wear scar surface, high graphite tribo-chemical film coverage, dangling bond passivation, and boundary lubrication.