Abstract: The operation principle and structure of a automatic fluorescent magnetic particle testing system for tyre surface defect detecting were introduced. The excited and emissive spectrums of the fluorescent magnetic particle were analyzed and researched. The digital image processing and identifying technique for automatic magnetic particle testing system are introduced in detail. The testing image acquired by the high precision CCD camera was proceeded one by one by such signal processing method as local average smoothness, edge extraction and binarization procession, after that the suspected defect could be recognized effectly, and it turned that defect automatic recognition of fluorescent magnetic testing system was possible.