The present invention provides a brightness compensation system and a brightness compensating method of an OLED display device, which comprises: an image pre-processing unit, an image-contrast control unit electrically connected to the image pre-processing unit, an aging compensation unit electrically connected to the image-contrast control unit, an aging-parameter detection unit electrically connecting the aging compensation unit and the image-contrast control unit, and an OLED display panel electrically connected to the aging compensation unit and the aging-parameter detection unit; which can prevent the brightness of the OLED display device from being degraded by the aging of the OLED display device, to ensure the display quality of the OLED display device, and to enhance the user experience and the product competitiveness.