The disturbance of clouds is an important factor in detection and simulation for sky, therefore, the study of cloud radiation is important. In different IR wavelengths, the properties of cloud can be much different, in order to study the apparent radiation of different wavelengths, the calculation of spectral radiation of cloud is required. An apparent radiation model for typical detection is presented in this article, and the calculations of radiation contribute to the cloud radiation is studied. The acquirement of parameters in the model is in the light of traditional two stream approximation, in additional, an experience formula of spectral optical depth is introduced, consequently, the infrared spectral radiation characteristics of low clouds in short and middle wave are gained, and apparent spectral radiation is calculated in middle and short wave ,and the calculation time is saved. Result shows that actual measurements and data from the model are consistent, hence the model is applicable as the data source in the detection and simulation in the cloudy background.