a contrast ratio promotion method for a display device is provided . the display device contains a back light module having a brightness and has a maximum luminance . the contrast ratio promotion method comprises the steps of continuously receiving a picture signal during each unit time , continuously calculating a ratio of a luminance of a picture corresponding to the picture signal to the maximum luminance , where the ratio is defined as an instant relative luminance , setting a first ratio , a first buffer time and a first adjusting time , and gradually reducing the brightness of the back light module to a minimum brightness during the first adjusting time when a first cumulated time for the respective consecutive plural instant relative luminances being smaller than the first ratio is longer than the first buffer time .