Aiming at the problem of testing the ABS gear ring round surface defectsby the traditional manual testingmethodswithlow efficiency, easy to error testing and missed testing,a method for testing the ABS gear ring round surface defect based on image processingisproposed.According to the actual produce, an on line visual testing system is designed and assembled. Using electric rotating machinery and gear ring supporting platform to drive gear ring around and get the picture of the gear ring round surface by liner CCD scanning. After processing the picture by image algorithm based on OpenCV,thedefectsarejudgedaccording to different existingarea,andthen the qualification of the gear ring is judged. Comparing with the results of system testing and artificial testing, itshows that the average testing time of each gear ring less than 4 s, the success rate of defect classification is more than 92%.