A white balance adjusting system for a display device includes a control member including a register storing white balance control commands and a preset white balance lookup table. A built-in self-test module is coupled to the control member and configured to store a test pattern. A first storage element is disposed in the display device. An optical sensor is configured to measure brightness and chroma data of the screen according to the test pattern. An external control device is operated by the control member to execute the white balance control commands so that white balance adjustment is implemented according to the preset white balance lookup table and the brightness and the chroma data of the screen, and a corrected white balance lookup table is produced. The corrected white balance lookup table is stored in the first storage element and provided to update the preset white balance lookup table.