The optical signals in the fiber optic gas sensor based on the opening cell are analyzed with the view of system. The criterion for designing the cell is put forward, i. e. the output property of the cell is equivalent to the White type cell under the case of no overlapping between the two nearest complex light beams on the mirror from which the cell cavity is made. A spike filter and a phase sensitive detector are utilized in the signal processing, in order that the logarithm value on the absolute voltage from the sensor system of the gas is measured. Thus, the gas sensor can be calibrated on the basis of the Lambert law. The cell can be used to measure atmospheric extinction coefficient at single wavelength.