In this study, we simulate the outgoing longwave radiation (OLR), radiative flux, and heating rate at each atmospheric level using the libRadtran radiative transfer code. The background gaseous absorption properties are taken into account on the basis of the correlated k-distribution method. The single scattering properties of cirrus clouds including the extinction efficiency, single-scattering albedo and asymmetry factor are parameterized with respect to effective particle size and wavelength. In this model, the atmosphere is divided into 100 layers and a thin cirrus clouds layer is inserted as one layer. The radiative heating rate is calculated as a function of the optical depth of thin cirrus clouds. Furthermore, the relationship of OLR and optical depth is investigated.