Abstract: Aiming at the temperature evolution problem in continuous groove milling of T800 carbon fiber reinforced polymer composites, experiments were conducted using a 10 mm-diameter diamond-coated fine-pitch nicked router. Tool, machined-surface, and chip temperatures were simultaneously measured by infrared thermography. A transient idle-travel cooling model was established using Newton’s law of cooling and the lumped-capacitance method, while surface thermal accumulation and chip cooling under different accumulation states were evaluated. The tool temperature at the end of cutting was mainly concentrated in the range of 380–430 ℃ and remained mainly at 220–260 ℃ after 24.75 s of idle travel. Cooling followed a first-order exponential decay; the rate constants obtained from endpoint and continuous data were 0.021 0 s⁻¹ and 0.022 4 s⁻¹, respectively, and the mean R² of the continuous-data fits was 0.998 7. The machined-surface temperature ranged from 70.00 to 135.87 ℃ and generally increased with cumulative cutting distance before stabilizing. The maximum temperatures of machine-table chips and surface-residual chips were 388.22 and 265.81 ℃, respectively, and greater accumulation was associated with slower cooling. These results provide a basis for thermal monitoring, machining-cycle optimization, and chip-removal control in continuous slot milling of CFRP.