An illumination device includes a light sensor, a light-emitting unit, a switch unit and a processing unit. The light sensor is configured for sensing ambient light intensity and providing results based on the sensed light intensity. The switch unit is configured for controlling the electrical connection between the light-emitting unit and a power unit. The processing unit includes a control module and a light adjusting module. The controlling module is configured for turning off the switch unit to cut off the electrical connection between the light-emitting unit and the power unit during the sensing of the light intensity. The light adjusting module is configured for adjusting the brightness of the light-emitting unit according to the results provided by the light sensor. A related method is also provided.