In order to reduce the infrared radiation of tank in actual combat, characteristics of tank barrel radiation after low speed firing are analyzed. Common SiO2 and Si are selected as the mediums according to the barrel radiation characteristic wavelength of 812 m. With the consideration of the dispersion relation, an insulating coating with photonic crystal structure is designed. Mathematical computation indicates that when the two mediums each have 4 layers and their geometric thicknesses are 1330 nm and 825 nm respectively, there will be a strict band gap of 812 m. When the geometric thicknesses of the two mediums increase, the band gap will have red shifts and the width will increase, and vice versa. As long as the geometry thickness variations of the two mediums are less than 10%, the original band gap will always exists. When the mediums are of 78 layers, the coating form a strict band gap in the above mentioned wavelength range. With the medium layer number increasing, there will be no substantial change with the band gap. The band gap structure is not sensitive to the change of incidence angle.