The metal surface crack under paint coating has the problem of low signalnoise ratio using conventional detection methods. To address this issue, an eddy current thermography detection technique based on the directional modulation principle is proposed. Firstly, the principle of direction modulation is introduced. By deducing the mathematical model, the feasibility of the method is proved. The modulation frequency is selected according to the theoretical formula. According to the directional modulation, the experimental platform is established. Then, the reason that the eddy current cannot rotate in full angle is analyzed. One kind of improved method based on PWM drive signals flipping is proposed. To solve the problem of resonance frequency drift caused by heating, a twochannel frequency tracking strategy is proposed. Finally, the artificial cracks of 45# steel under different paint coating thicknesses and the cracks of 20# steel weld under 0517 mm paint coating thickness are detected and compared with the results of magnetic powder detection. Detection results show that the direction modulation method is competent to detect the artificial cracks under 0902 mm paint coating and the simulated natural cracks under 0517 mm paint coating. Compared with the magnetic particle detection results of the artificial cracks under 0486 mm paint coating and the simulated natural crack under 0451 mm paint coating, the directional modulation method effectively improves the detection ability of under paint coating cracks.