Abstract: For surface crack defects of ferromagnetic materials, the late-model eddy current thermography method was used for testing research. The mechanism of heating in crack caused by eddy current excitation was analyzed, the eddy current excitation device based on series resonance was designed, and the eddy current thermography testing system which was in high frequency and low power was constructed. Through the designed test system, ferromagnetic materials were tested. In view of the effect of “transverse fuzziness” in the image sequence, the enhancement method of high-pass filtering and sharpening on the original heating image, the histogram twin peaks method, the iterative method and the watershed method based on mathematical morphology for the image segmentation were adopted, and the quantitative identification of crack length was finally realized. Experimental results have shown that eddy current thermography can realize a quick, efficient and intuitive detection on crack defects of the specimens, and when the length of crack is 5mm, the error is less than 15 percent.