Based on the theory of evanescent wave and optical resonance, the existence form of evanescent wave in photonic crystal and the refractive index sensing mechanism of air gate photonic crystal F-P cavity are studied, and the relationship model between the resonant wavelength and the detected gas refractive index is established. When the incident light travels through the photonic crystal at a certain angle, which is larger than the critical angle of total reflection, as a result of the action of evanescent wave, an F-P cavity is formed in the center of dielectric layer and produces resonance. The electromagnetic field is local enhanced, and contacts with detected gas sufficiently, which causes the refractive index sensing structure has a high sensitivity with the detected gas refractive index. The transfer matrix theory is used to numerical simulation, the results show that the Q value can attain 3447.0 and the sensitivity can attain 1260.0 nm/RIU. So the proposed photonic crystal F-P cavity refractive index sensing structure has good sensing properties, and provides certain theoretical references for the design and application of high precision gas refractive index sensors.