A maintenance method of a gas detecting device includes the following steps: providing the gas detecting device that includes a shell component, a sensing component, a dust blocking element, and a detecting area, the sensing component including a sensor, a reference light source, and a processor, and the dust blocking element including a cover; the reference light source generating a reference beam projecting toward the cover and the detecting area, and the reference beam generating a feedback beam projecting toward the cover and the sensor through being reflected by a part of the shell component positioned at the detecting area; the sensor receiving the feedback beam to generate a measured information; and the processor comparing the measured information to an initial information in the processor, so as to obtain a pollution degree information of the cover.