Micro scale grating has distinctive advantages and broad application prospects in controlling spectral emissivity of material. Based on rigorous coupled wave analysis (RCWA) and electromagnetic parameters, the spectral selective property of the emissivity of micro scale grating of steel in infrared band is investigated. Furthermore, combined with field distribution in grating region, the physical mechanism to form spectral and polarization selective properties is analyzed. The analyzed structure has guidance signification in the applications of infrared emissivity control.