1. A method for adjusting a skin color of a digital image, adapted to adjust the skin color of an input image, the method comprising: loading the input image; performing a skin color detection process on the input image to generate a skin-color probability plot Sp in a size corresponding to the input image; providing a hue-saturation lookup table named LUT_Color; performing a skin color reproduction process on the input image to look up the LUT_Color for a chrominance pixel value of a corresponding color for each pixel value of the input image to generate a first image; adjusting each pixel value of the first image by using the skin-color probability plot Sp to generate a second image, comprising: obtaining a first blue chrominance image Cb a and a first red chrominance image Cr a ; respectively performing a calculation at a corresponding ratio on each pixel in the first blue chrominance image Cb a and the blue chrominance image Cb according to the skin-color probability plot Sp, to output a second blue chrominance image Cb c , wherein 
 Cb c ( x,y ) =Sp ( x,y )× Cb a ( x,y )+(255 −Sp ( x,y )× Cb ( x,y )), and (x,y) is position coordinates of a pixel; respectively performing a calculation at a corresponding ratio on each pixel in the first red chrominance image Cr a and the red chrominance image Cr according to the skin-color probability plot Sp, to output a second red chrominance image Cr c , wherein 
 Cr c ( x,y ) =Sp ( x,y )× Cr a ( x,y )+(255 −Sp ( x,y )× Cr ( x,y )), and (x,y) is position coordinates of a pixel; and generating the second image according to the second blue chrominance image Cb c and the second red chrominance image Cr c ; performing a skin color smoothing process on the second image to generate a third image; and performing an image mixing process to mix pixel values of the input image and the third image at a corresponding mixing ratio to generate a target image.