Embodiments of the present disclosure provide a device and a method for adjusting brightness of a display device. In the method for adjusting brightness of the display device, a first voltage data is applied to the display device when a peak brightness condition is met, or a second voltage data is applied to the display device when the peak brightness condition is not met, enabling the first voltage data greater than the second voltage data to be applied to the display device when the peak brightness condition is met, thus increasing the brightness when the display device emits light to display an image to be displayed and improving contrast.