A method for fabricating high aspect ratio (HAR) metallic gratings using nanoimprint together with sputtering and reactive-ion etching (RIE) was introduced. The reflection spectrum is measured by Fourier transform infrared (FTIR) spectrometer in the mid-infrared (Mid-IR) region. The reflection peaks will appear just when the p-polarized light incident normally to the grating vector direction, which is very similar to the phenomenon of surface plasmon resonance. This is the so-called spoof surface plasmon resonance (SSPR). Theoretical analysis based on rigorous coupled wave showed that spoof surface plasmon resonance is very sensitive to the change of refractive index in the surface of the metal. Thus this phenomenon has its potential use as a refractive index sensor. In addition, the shift of resonance wavelength with the refractive index of the metal surface is completely linear. The refractive index sensitivity of the Mid-IR SPR sensor are predicted to be 1600 nm per refractive index unit (1600 nm/RIU) and 5000 nm/RIU for the positive and negative order diffractive waves, respectively. The corresponding figure of merits of the whole system is predicted to be 20 RIU-1 and 60 RIU-1, respectively. The list of applicable target materials will certainly expand greatly if mid-IR SPR-based sensors are developed.