The disclosure discloses a backlight brightness adjusting method and device and a liquid crystal display device. In the backlight brightness adjusting method, grayscale characteristic values of respective primary colors in an image to be displayed are obtained, that is, contributions of the respective primary colors to an overall grayscale are obtained, to thereby obtain the grayscale contributions of the respective primary colors in the image to be displayed respectively; a backlight value is determined by giving different coefficients to the grayscale characteristic values of the respective primary colors; and finally an output of a backlight driver is adjusted according to the backlight value to thereby adjust backlight brightness.