The traditional filter demodulation sy stem has lower demodulation accuracy and the filter′s curve is not strictly linear.This paper deduces theoretically that the ratio of the filtered light which exists in the linear filter system and the direct probe light′s optical power is proportional to the filter′s transmittance function.In order to overcome the problem with t raditional demodulation method of linear filter the nonlinear demodulation processing in accordance with the relationship between wavelength and ratio has greatly improved the performance of present li near filter dem odulation system.The experimental results show that temperature demodulation accuracy of the nonlinear demodulation system is h igher than that of the linear demodulation system.What′s more,this paper i ntroduces the concept of virtual wavelength to the nonlinear filter demodulation system,which succeeds in directly demodulating the environmental temperature changes without calibrating the chan ges of concrete wavelength.