The wavelength-selection trapezoid gratings which can improve the infrared absorptance contrast are demonstrated. The spectral absorptance of silicon trapezoid grating is studied by using the rigorous coupled-wave analysis (RCWA) numerical scheme. The optical performance of the trapezoid gratings with different side-wall slopes on the groove ridge is calculated and analyzed. According to the analyzed results, the side-wall slope of the trapezoid grating plays an important role for enhancing the contrast of the infrared absorptance. It is found that the absorptance peak is located at the wavelength of 7.0 μm, which is equal to the period of the trapezoid grating. The maximum spectral absorptance contrast is given at the proper side-wall slope of groove ridge of the trapezoid gratings. The influences of the oblique incident light are also analyzed. When the incident angle increases, the absorptance peak deviates from the original location of the normal incident condition. The absorptance peak not only decreases its magnitude but also be split.