At present, the method we often use to measure the parallax of Large-aperture collimator is pentaprism method. Since artificial reading caused error, a kind of automatic inspection system for parallax of large-aperture collimator was put forward based on circular matching and data fusion. A high-definition camera was used to collect the image for large-aperture collimator. The center of the circle was found through OTSU, filtering, edge detection, circular catching and data fusion. A calculation result for parallax was given finally. In the process of calculating parallax, it can do more testing, so the curve of the parallax can been given out, which will make the collimator calibration convenient. Test results of automatic inspection system for parallax are proved to be accurate, and the automatic inspection system has good repeatability.