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Herein, based on the finite‐element calculation method, a high‐sensitivity infrared absorption sensor is designed with ZnSe as the substrate, Mo as the waveguide layer and periodic grating, and Ge as the thin film. The absorption performance of the structure is not sensitive to the polarization of the incident light when the transverse‐magnetic light wave is perpendicular to the structure. The thickness of the Ge thin film and the width of the grating column are the main factors for determining its absorption and sensing performance. The optimized structure is sensitive to the refractive index on the grating. Herein, a correspondence between the refractive index change and the position of the peak of the absorbance spectrum is established. When this structure is used as a refractive index sensor, the sensitivity can reach 2300 nm RIU−1 with a high figure of merit of 37.09. In addition, temperature sensing in ethanol shows that the waveguide can quantify the solution temperature by resonance. It is demonstrated in the results that the grating absorber has great potential applications in biomedical sensing.