A method for adjusting color saturation is adapted for color adjustment of a pixel in a color space. The method includes: determining a color cube in the color space; selecting one from a plurality of diagonals of the color cube as a primary diagonal, and setting the primary diagonal at a vertical axis of the color space; determining a hue azimuth angle and a height in the color cube desired by the pixel; determining a reference point at an outermost periphery of the color cube corresponding to the azimuth angle, and obtaining a reference height and a reference horizontal distance of the reference point distant from the primary diagonal; and obtaining a color saturation value of the pixel by multiplying the reference horizontal distance with a ratio between the reference height and the height of the pixel.