Abstract: In order to solve the problem of low detection accuracy of Mark circle in traditional printed circuit board (PCB), the curve fitting and optimized fitting algorithm were used to improve the Mark circle detection mode, and the theoretical analysis was carried out. The test platform was built by using microsoft foundation classes (MFC) to embed the standard machine vision algorithm package HALCON for experimental verification. The results showes that the success rate of the method was 97%, the detection accuracy was less than 0.3pixel, and the detection time was less than 100ms. It solved the problem of low accuracy of Mark circle in traditional PCB. At the same time, a large number of test data showed that it could still ensure a good detection effect in the environment where the image was panned, rotated, and zoomed. This research has certain reference to the actual PCB production detection.