1. A saturation compensation method, which is executed by a processor, the method comprising: retrieving an input image; performing at least one first image process on the input image to generate a first image, wherein the at least one first image process lowers saturation of the input image to generate the first image; calculating saturation corresponding to each pixel in the input image; converting the input image and the first image to a color space, wherein the color space comprises at least one color channel; calculating the saturation corresponding to each pixel in each color channel of the input image in the color space respectively, wherein the saturation corresponding to each pixel of the input image is lower, the saturation corresponding to each pixel of the output image has a higher weighting; and performing a saturation compensation process on the first image according to a first formula to generate an output image, wherein the first formula is expressed as: O i ′ ⁡ ( x , y ) = p ⁡ ( x , y ) × O i ⁡ ( x , y ) + [ 1 - p ⁡ ( x , y ) ] × Y O ⁡ ( x , y ) Y I ⁡ ( x , y ) ⁢ I i ⁡ ( x , y ) ; where p(x,y)=f(S I (x,y)), and I i denotes the amount of ith primary stimuli in a color space of the input image; O i denotes the ith color channel of the first image; O′ i denotes the ith color channel of the output image; Y o denotes brightness information of the first image; Y I denotes brightness information of the input image; (x, y) denotes the coordinates of a pixel in an image; and p denotes a weighting factor.