1. A compensation method for alleviating color shading in a digital image, adapted to correct a color shading phenomenon in a digital image that causes luminance differences between regions in the digital image, the compensation method comprising: providing a uniform color block, and capturing a color block image of the color block; cutting the color block image into a plurality of horizontal segments in a horizontal direction, and calculating horizontal compensation coefficients corresponding to the horizontal segments, respectively; performing an interpolation process to linearly interpolate the horizontal compensation coefficients, so as to generate a plurality of horizontal interpolation coefficients; finding out corresponding horizontal compensation coefficients of the same horizontal segments according to the horizontal interpolation coefficients, and dividing each of the horizontal interpolation coefficients by the corresponding horizontal compensation coefficients to obtain a plurality of horizontal correction coefficients; multiplying the horizontal correction coefficients by corresponding horizontal interpolation coefficients, respectively, so as to obtain a plurality of determination horizontal correction coefficients, and obtaining determination horizontal correction coefficients corresponding to all the horizontal segments, respectively, by linearly interpolating any two adjacent determination horizontal correction coefficients; cutting the color block image into a plurality of vertical segments in a vertical direction, and calculating vertical compensation coefficients corresponding to the vertical segments, respectively; multiplying the determination horizontal correction coefficients by the vertical compensation coefficients, respectively, to obtain a color shading compensation coefficient of each segments of the color block image, respectively; and multiplying an average luminance value of each segment of the color block image by a corresponding color shading compensation coefficient, respectively, thus compensating for color shading in the color block image.