Abstract: Geothermal energy is a clean and ecologically friendly energy source with significant potential. The temperature variations between the Polycrystalline Diamond Compact (PDC) cutter and the rock of the reservoir are the key factors affecting the cutting performance when drilling through formations with thermally damaged rock. To better investigate the temperature rise, a series of rock samples treated at high temperatures (9–300 °C) were broken with a PDC cutter. The performance of the PDC cutter on these samples was studied using cutting force sensors, high-speed photography, and the thermal infrared imager. Based on the experimental data, a new cutting force evaluation parameter, η, is suggested. The link between the cutting force and rock properties is discussed in detail. The present results indicate that the average cutting force of high-temperature-treated granite is 3–5 times that of the thermally damaged sandstone. Furthermore, a critical temperature for thermal damage has been identified in granite cutting at 100–200 °C and in sandstone at 100 °C. This corresponds to the temperature at which interlayer water loss and thermal crack closure occur. The results also indicate that when the treatment temperature exceeds the critical threshold, both the cutting force and temperature rise exhibit more significant changes with increasing temperature. Additionally, the maximum temperature of the PDC cutter during granite cutting can reach 47.6 °C, which is almost 34 °C higher than that of sandstone. Regarding debris size, granite is much less sensitive to the treatment temperature, showing only slight changes in debris size compared to sandstone as the treatment temperature increases. The increasing cutter-rock interface area can significantly reduce frictional heat generation while increasing the cutting force and enhancing the temperature rise. The parameter valuation of the newly defined parameter η, which is related to frictional heating, shows that the capacity of the thermal generation and the heat transfer change as the temperature rises at the cutter-rock interface. At last, the correlation analysis indicates that the cutting force of sandstone and granite is highly correlated with σt2/σc, E/σc and σt. This study serves as a theoretical support and technical guidance for cutting hot dry rock (HDR), which is of great significance to HDR drilling.