Based on the local characteristics of photonic crystals and the optical sensing principle of porous silicon, the mirror symmetrical porous silicon photonic crystal index sensing structure is proposed. The sensing theoretical model is obtained based on layered transfer matrix method. The relationship between the resonant wavelength and the structural parameters of the porous silicon photonic crystal is deduced. By adjusting the structure parameters of porous silicon for both high and low refractive index layers, the full width at half maximum of the band gap will be narrowed and the quality factor (Q value) can be improved. Based on the transfer matrix method, when the methanol vapors with different concentrations enter the porous silicon photonic crystal index sensor structure, the theoretical simulation is performed using MATLAB. The relationship model between the change of the resonant peak wavelength and the variation of the effective refractive index for the porous silicon layer is established, and the refractive index sensing properties are analyzed in the numerical simulation. The simulation results show that the Q value of the refractive index sensing structure is 3114.75 and the sensitivity can attain to 903.9 nm/RIU, which demonstrate the effectiveness of the sensing structure, and it can provide certain practical reference value for the design of high Q and high sensitive refractive index sensors.