According to ratios of filter light and reference light and the relationships of wavelength, changes of external physical quantities are determined caused by changes of ratios. It is the principle of FBG demodulation of linear filters. For traditional filter demodulation system, the relationships between wavelength and ratios are processed in linearity. And curves of linear filters are not entirely linear. In the experiment, deduced ratios in theory are directly proportional to the transmissivity of filters. Temperature, ratios corresponding to strains and wavelength are processed separately. When a strain is zero and temperature changes, a high-order item is added during data fitting process, wavelength and ratios are processed in nonlinearity. When the temperature is a constant but pressure changes, wavelength and ratios are processed in linearity. Fitting results show that curves of wavelength and ratio are similar to filtering curves and wavelength differences are equal corresponding to the same ratios at different temperature curves.