An optical fiber gas sensing method was proposed based on the detection of light sensitive phase of light wave combined with organic polymers and the experimental verification was given. When using different concentrations of acid gases to act on the organic polymers, the refractive index of the organic polymer will change. By filling organic polymer film in optical fiber F-P cavity and analyzing the spectral characteristics of fiber F-P cavity output, it can be realized the hydrogen sulfide gas sensing. Experimental results show that accompanied by the increasing of concentration of the gas under the test, the refractive index of organic polymer decreasing. At low concentration, a record (0.726~1.006)×10－2 cm/%VOL of system resolution is obtained, Phase sensitivity is found to be 1.276×10－2 rad/%VOL and concentration resolution is found to be 0.078ppm. Through this test, it is proved that the possibility of gas sensing by using organic polymer. This system can be applied to high-precision measurement of carbon dioxide, hydrogen sulfide and other gases in the petrochemical field.