A part error visual judgment system comprises a light path conversion device for concentrating light rays reflected by different parts of a part on a conveying belt into one window, so that a camera system only uses all pixels to shoot the window, and the actual physical length represented by each pixel point is greatly reduced; a RGB color space-based color image edge extraction system used for extracting an image edge according to a picture shot by the camera system and calculating a part size; and a judgment system used for judging whether the part is qualified and giving out a corresponding signal. The light path conversion device capable of improving the part size detection precision and a new RGB color space image edge extraction algorithm are utilized, and software adapting to the algorithm can be compiled, so that the part size visual detection precision is improved, the cost of a visual detection system is reduced, and the method is particularly suitable for small and medium-sized enterprises.