In order to design a gas sensing structure with superior performance, a hybrid waveguide structure consisting of a subwavelength dielectric grating, a dielectric layer and a metal substrate was proposed. Based on the guided mode resonance, four narrow-band resonance defect peaks were formed in the reflection spectrum. Porous silicon was used as the waveguide layer. Based on the refractive index tunable properties of porous silicon, the structure can realize the dynamic monitoring of sample gas concentration by observing the drift of the resonance wavelength, and the analysis of sensing characteristics of the structure. The results illustrate that the sensitivity of three narrow-band resonance peaks are 300, 350, 400 nm/RIU respectively. And the figure of merit can reach 120, 152.2, 307.7/RIU respectively.The structure has a high detection accuracy and can be used for gas sensing.