Abstract: To study the infrared characteristics of clouds with different thicknesses in different wavebands, a method for calculating infrared radiation of cloud based on optical depth is proposed. The infrared model of cloud was established after studing each infrared radiative component. According to the relation of infrared characteristics to waveband and thickness, the emissivity, reflectivity and transmissivity were computed respectively in middle infrared and long infrared wavelength, and the radiance of cloud was computed accordingly. Result shows that the radiation of cloud is a funtion of optical depth, and the actual measurements and the data from the model are consistent. It is proved that infrared radiation calculation of cloud based on optical depth can be used as the data source in infrared characteristic detection, analysis and simulation for the cloud background.