Abstract: The working principle, the structure and the digital image processing technology of a fluorescent magnetic particle testing defect automatic recognition system were introduced in this paper, including extracting the original image from CCD camera, median filtering, edge detection and image binarization. Moreover, the optical structure was analyzed, the physical characteristics, excitation spectra and emission spectra of the fluorescent magnetic powder and the lighting methods of light source were included. The system can effectively identify the circular defects of 0.5 mm diameter and the linear defects of 1.5mm in length.