The present disclosure provides a backlight module, a display device and a method for adjusting brightness of the backlight module. The backlight module includes a light guide plate comprising a light emitting surface and a first surface opposite to the light emitting surface, and a reflective component on a side of the light guide plate away from the light emitting surface, the reflective component having a reflectivity adjustable to adjust a brightness of the backlight module. In this way, the reflective component in the backlight module can be used to dynamically adjust the brightness of the backlight and improve the contrast between different regions of the backlight module, thereby realizing the display effect of the high dynamic range image. Moreover, the backlight module is low in cost, simple in preparation, and suitable for various types of display devices.