1. A color gamut mapping method based on a color gamut of a source image, comprising following steps: step (a) inputting a source image, measuring a grayscale value of each of color sub-pixels corresponding to each pixel point; step (b) determining multiple preset target pixel points on the source image based on the grayscale value of each of the color sub-pixels which has been measured; step (c) calculating multiple coordinate values in a uniform chromaticity space corresponding to each of the multiple preset target pixel points; step (d) determining a color gamut of the source image based on the multiple coordinate values; step (e) extracting a color gamut boundary of a target device in order to obtain a color gamut of the target device; and step (f) performing a color gamut mapping between the source image and the target device.