We demonstrate the nonreciprocal optical phenomenon of SiC gratings on substrate in infrared band, in which the Lorentz-Drude equations of dielectric constant tensor are proposed to describe the nonreciprocal optical properties as magnetic field applied on the magneto-optical materials, under variable intensity and wavelength. Moreover, the properly designed geometrical factors are proposed, and the good nonreciprocal absorption properties of SiC in thermal radiation wavelength band are presented. The dependence of the absorptivity as a function of different structure parameters, such as thickness of different layers, filling ratios, is studied in details. Furthermore, the electric field intensity is also presented for understanding light coupling, propagation. Numerical evidence shows that the nonreciprocal absorption performance is sensitive to the incidence angle, as well as the magnetic field strength. The relative study is useful to the thermal radiative design in photovoltaic and optical instrument.