Digital average filtering is a widely used signal processing method.In small signal detection,digital average filtering can obtain higher signal-to-noise ratio and can improve the detection sensitivity.This paper puts forward a new method using optical device to realize the digital average filtering.Basing on difference absorption principle,a new optical fiber gas sensor system based on a scanning light source is presented,and a novel optical gas sensor system is designed.By utilizing ETALON and reference grating,it can provide reference wavelength,and realize the spectrum reproducing of the output signal.Therefore,the same spectral signals can be superposed.This method is able to overcome the uncertainty effect caused by the transmission wavelength of tunable Fabry-Perot filter and filter′s driving voltage.Experimental results show that the gas detection sensitivity is remarkably improved and can detect fairy low concentration acetylene gas.The system is suitable for industrial gas monitoring.