1. A method for controlling low grayscale white balance, wherein a low grayscale is less than or equal to a grayscale threshold n max value, the method comprising: measuring a tristimulus value of a grayscale zero and a tristimulus value of the grayscale threshold value n max to calculate a total color difference value ΔE(n max −0) between the grayscale zero and the grayscale threshold value n max ; determining a color difference relationship among grayscale from the grayscale zero to the grayscale threshold value according to the total color difference value ΔE(n max −0) between the grayscale zero and the grayscale threshold n max , and the color difference relationship makes eyes observe that variation from the grayscale zero to the grayscale threshold n max is smooth; controlling white balance according to the color difference relationship between the grayscale zero to the grayscale threshold n max ; wherein, the color difference relationship is ΔE, the ΔE is ΔE(n max −0)/(n max −0); or the color difference relationship is ΔE(m), the ΔE(m) is f(ΔE(n max −0)/(n max −0)), wherein, the f is a monotonically increasing function which makes the ΔE(m) increased with the grayscale m, the ΔE(m) is a color difference between the grayscale m and the grayscale m−1.