Atmospheric downward radiation is a key parameter in surface radiation and energy balance. It plays a key role in retrieving surface temperature and emissivity and in the study of climate change. Based on the analysis and comparison of the existing atmospheric downward radiation measurement methods, the research of atmospheric downward longwave radiation of Hefei, China, is carried out. Hefei downward atmospheric radiant flux is simulated and calculated by using MODTRAN radiation transmission model, and then taking the atmospheric downward radiant flux obtained by this method as benchmark, the performance of the widely used sunny day empirical model is evaluated, and the applicability of Idso model and ngstrm model to Hefei atmospheric conditions is verified. In addition, the parameters in the Idso model are modified by using MODTRAN radiation transmission model simulation data to improve the simulation accuracy of the model. This study provides a reliable method for conveniently, quickly and accurately obtaining Hefei atmospheric downward radiant flux.