1. A white balance adjustment method of an image capturing device, the method comprising: S 1 : capturing a raw image using the image capturing device; S 2 : setting an initial maximum threshold value R max and minimum threshold value R min of each R, G, and B channels of the raw image, making R max =R min , wherein an average value of color scale within the maximum threshold value represents a reference white, and an average value of color scale within the minimum threshold value represents a reference black; S 3 : extracting color scale values of each R, G, and B channels of the raw image, and obtaining a minimum value “a”, a maximum value “b”, and an average value “c” of the color scale values corresponding to the channels; S 4 : defining a modulation factor “r”, r=(c−a)/(b−a) of each R, G, and B channels; S 5 : defining a new maximum threshold value R max ′ and a new minimum threshold value R min ′ of each R, G, and B channels; if r<0.5, then R min ′=r×R min and R max ′=(2.0−r)×R max ; on the contrary, R min ′=(2.0−r)×R min and R max ′=r×R max ; and S 6 : obtaining an average value of the color scale within the new maximum threshold value R max ′ and the new minimum threshold value R min ′ of each R, G, and B channels; and obtaining the reference black and the reference white corresponding to the channels; according to the reference black and the reference white of R, G, and B channels, a color histogram stretching (CHS) method is adapted to adjust the white balance of each corresponding channels.