Based on the resonance principle of waveguide coupled gratings and optical sensing properties of porous silicon, a refractive index sensing model of waveguide coupled gratings nano-heterostructure is proposed. According to the theory of Goos-Hnchen displacement and phase matching conditions of waveguide coupled gratings resonance, the relationship model between the resonant wavelength and the refractive index of the detected sample is established, and the refractive index sensing properties of the refractive index sensing structure are analyzed. Taking the porous silicon as bearing units of detected samples according to its effective bearing mechanism, we analyze the sensitivity and the quality factor Q of the refractive index sensing structure by detecting solutions of ethylene glycol with different concentrations. The results show that the sensitivity of the refractive index sensing structure to the solution of ethylene glycol is 2.2 nm/1%, and the Q value is 226, which proves the validity of this sensing structure. The sensing structure can detect the samples with low concentration.