In a method for marking discrepancies of a captured image of an object, an image is captured and compared to a standard image. A discrepant image showing any discrepancies of the captured image is generated, and is separated into an R grayscale image, a G grayscale image, and a B grayscale image. An R channel matrix group, a G channel matrix group, and a B channel matrix group are created. R channel negative matrixes, G channel negative matrixes, and B channel negative matrixes are determined from the RGB channel matrix groups. RGB pixel groups are calculated based on the R channel negative matrixes, the G channel negative matrixes, and the B channel negative matrixes. A target pixel group to be marked is determined by calculating an intersection of the RGB pixel groups. The discrepancies of the digital image are marked out according to the target pixel group.