A photodetecting device and method of using the same are provided. Light is used to irradiate the optical filter layer of the photodetecting device and positions of the electrons and the holes in the polycrystalline silicon nano-channel layer are rearranged by the light with a wavelength range capable of passing through the optical filter layer. The current between the source and the drain is changed by rearranging the positions of the electrons and the holes, so as to generate a current difference. The intensity of the light is calculated by the current difference.