Abstract: Eddy current testing technology was used to detect the thermal damage of carbon fiber composites. Three damage points with different degrees of thermal damage were tested and compared at 1.00, 2.00, 3.33 and 5.00 MHz detection frequencies, respectively. The normalized impedance curve of the detection coil at each frequence was collected and decomposed in X and Y directions. The thermal damage area was identified with the change of signal amplitude. Furthermore, the eddy current testing results were verified by immersion ultrasonic C-scan testing. The results show that the degree and area of thermal damage have obvious effects on impedance in eddy current testing. The more serious the thermal damage, the greater the change of the impedance curve amplitude. Furthermore, the larger the thermal damage area, the wider the curve. The frequency variation changes the phase of impedance curve, but has little effect on the detection sensitivity. Although there are errors in the measured results of heat loss area, they are close to those from immersion ultrasonic C-scan testing. This work is of reference value for conducting thermal damage assessment of carbon fiber composites.