The invention relates to the technical field of display and discloses a device and method for adjusting screen brightness of a splicing display device, which are used for adjusting the uniformity of the screen brightness of the splicing display device. The adjusting device comprises an adjusting module, an acquisition module and a processing module, wherein the adjusting module is used for controlling a light-emitting unit in a backlight module in the splicing display device to rotate in a pre-set range; the acquisition module is used for acquiring brightness information of a splicing screen when the light-emitting unit rotates to each angle; the processing module is used for calculating the specific value of the minimum brightness value and the maximum brightness value in each piece of the brightness information, and determining the maximum value in the plurality of specific values, and the specific rotary angle of the light-emitting unit corresponding to the brightness information with the maximum value; and the adjusting module is controlled to rotate the light-emitting unit to the specific rotary angle.