There is provided a portable electronic device including a backlight module, an ambient light sensor, a proximity sensor and a processing unit. The backlight module illuminates with backlight brightness. The ambient light sensor is configured to detect ambient light intensity. The proximity sensor is configured to detect an object. The processing unit is configured to activate the proximity sensor when the ambient light intensity detected by the ambient light sensor is lower than a predetermined value or decreases more than a predetermined range, and to maintain or reduce the backlight brightness according to a detection result of the proximity sensor. There is further provided an automatic detection method.