An automatic calibration system for a traffic system includes at least one position determining device, at least one image capturing device, a matching and tagging module, an image analysis module, and a calibration module. The position determining device detects a vehicle in violation of traffic rules and activates the image capturing device to capture images of the vehicle. The vehicle is matched and tagged in the images according to vehicle-related information detected by the position determining device by the matching and tagging module. The image analysis module analyses a plurality of images selected from the images to obtain an analysis result. The analysis result is compared with the vehicle-related information by a processor. If the analysis result is different than the vehicle-related information, the position determining device is calibrated by the calibration module. If the analysis result is the same as the vehicle-related information, a calibration is not performed.