Based on the photonic local feature of photonic crystals, one-dimensional mirror air gate photonic crystal structure is proposed. Due to the introduction of mirror structure, a defect cavity is formed in the middle of the photonic crystal, and then the resonant transmission peak can be obtained in the forbidden band of transmission spectrum. The transfer matrix method is used to establish the relationship model between the resonant transmission peak and the structure parameters of the photonic crystals. Using the rectangular air gate photonic crystal structure, the dynamic monitoring of the detected gas sample parameters can be achieved from the shift of the resonant transmission peak. Taking formaldehyde as the detected sample, the Q value and sensitivity of the refractive index sensor are analyzed. The simulation results show that the Q value can attain to 1739.48 and the sensitivity can attain to 816.67 nm·RIU-1 (RIU is refraction index unit), which demonstrates the effectiveness of the sensing structure. The structure can provide certain theoretical reference for air pollution monitoring and gas component analysis.