Disclosed are a display device and a method for adjusting its display brightness. The display device includes a display screen and an ambient light sensor in the display screen, wherein the ambient light sensor includes a photodeformable element, and the photodeformable element includes a photodeformable material layer; and the photodeformable element is configured to deform in response to a change in ambient light to obtain output of the ambient light sensor. The ambient light sensor has a simple structure and is easy to be made and to be combined with the display screen.