The algorithm of infrared signature of aeroplane along the line of sight is introduced. The aeroplane model is simple, helping to get the projective area of aeroplane along the line of sight. In the calculation of infrared intensity, not only infrared radiation sources including skin, spout and plume are considered, but also the effects on the infrared radiation exerted by the background radiation of the sun, the sky, the earth and the atmospheric attenuation are considered.