The invention provides a backlight brightness control method and device and a liquid crystal display device. The method comprises the following steps: determining basic backlight data and expansion backlight data of a backlight in each backlight subregion corresponding to a subregion image according to the gray scale data of the subregion image, wherein the basic backlight data is used for indicating the duty ratio of pulse width modulation (PWM) of the backlight in the backlight subregion, and the expansion backlight data is used for indicating the peak current gain coefficient of the backlight in the backlight subregion; and adjusting the duty ratio of the PWM of the backlight in the backlight subregion according to the basic backlight data, and adjusting the peak current of the backlight in the backlight subregion according to the expansion backlight data to control the brightness of the backlight in the backlight subregion. Therefore, dynamic change range of adjustment of the brightness of the backlight is improved, and better image region contrast is realized and the dynamic range is displayed better.