A wearable electronic device comprising a wearable component and a device body is provided. The device body is connected to the wearable component. The device body includes a processor, a main light source, an auxiliary light source and a light sensor. The light sensor is electrically connected to the processor. An absorptivity to main rays from the main light source by human blood is more than twice of the absorptivity to auxiliary rays from the auxiliary light source. The light sensor detects intensities of the main rays and the auxiliary rays reflected by the human body. The processor corrects an error of the intensity of the main rays received by the light sensor according to the intensity of the auxiliary rays received by the light sensor to obtain a heart rate. A method for detecting a heart rate is also provided.