A subwavelength metal grating/dielectric/metal hybrid waveguide sensing structure is proposed. In the structure, the resonance mode of guided mode, surface plasmon polariton resonance mode and local surface plasmon polariton mode can be generated, and these modes are coupled with each other, resulting in two narrow band resonance defect peaks in the reflection spectrum. After studying the magnetic field distribution at the resonance defect peaks and the influence of structural parameters on the reflection spectrum, we analyze the resonance modes in the structure. According to the principle of mode resonance of guided mode and surface plasmon resonance, the relationships between the wavelength of resonance defect peaks and the structural parameters are established. The drifts of resonance peaks wavelengths are observed, and the dynamic real-time monitoring of the gas samples can be realized. The porous silicon is used as the sensing carrier, and the formaldehyde is used as the sample to be tested. The quality factor and sensitivity of the sensor are analyzed. The results shows that the quality factors of two narrow band resonance defect peaks are 42.3 RIU-1 and 78.5 RIU-1, and the sensitivities are 466 nm/RIU and 628 nm/RIU, respectively.