Abstract: In drilling ultra-deep wells, the drilling fluid circulation usually causes erosion damage to downhole casing and drilling tools. However, the extent and process of this damage to the downhole tools is intricate and less understood. In order to systematically evaluate and clarify this damage process for different types of drilling fluid contamination, this research uses a high-temperature drilling fluid damage device to simulate the damage caused to the casing/drilling tools by various drilling fluid under a field thermal gradient. The results show that the drilling fluid residues are mainly solid-phase particles and organic components. The degree of casing/tool damage decreases with an increase in bottom hole temperature, and the casing/tool is least damaged within a temperature range of 150–180 °C. Moreover, the surface of the casing/tool damaged by different types of drilling fluid shows different roughness, and the wettability of drilling fluid on the casing/tool surface increases with an increase in the degree of roughness. Oil-based drilling fluid have the strongest adhesion contamination on casing/drilling tools. In contrast, polysulfonated potassium drilling fluid and super-micro drilling fluid have the most potent erosion damage on casing/drilling tools. By analyzing the damage mechanism, it was established that the damage was mainly dominated by the abrasive wearing from solid-phase particles in concert with corrosion ions in drilling fluid, with solids producing many abrasion marks and corrosive ions causing a large number of pits. Clarifying drilling fluid's contamination and damage mechanism is significant in guiding the wellbore cleaning process and cutting associated costs.