Accuracy of the upward longwave radiation data of GEWEX-SRB (Global Energy and Water Exchanges Project-Surface Radiation Budget), ISCCP-FD (International Satellite Cloud Climatology Project-Flux Data) and CERES-SYN (Clouds and the Earth′s Radiant Energy System-Synoptic Radiative Fluxes and Clouds) in polar regions is studied. In the experiment, two kinds of ground observation data of BSRN (Baseline Surface Radiation Network) and CEOP (Coordinated Energy and Water Cycle Observations Project) in polar regions are used as reference data, and the downscaling method is used to deal with them. Finally, accuracy of the radiation products is evaluated. Research results show that the overall accuracies of the three kinds of commonly used upward longwave radiation data are lower in polar regions. Absolute values of the root mean square error (RMSE) and the mean absolute error (MAE) are more than 15 W·m-2. The mean RMSEs of GEWEX-SRB, ISCCP-FD and CERES-SYN are 23.70 W·m-2 (8.69%), 25.14 W·m-2 (9.62%) and 22.98 W·m-2 (8.80%), respectively, and the mean MAEs are 18.53 W·m2 (6.96%), 20.09 W·m2 (7.70%), and 17.73 W·m2 (6.79%), respectively. Through the analysis of accuracy, the factors that affect the accuracy of the upward longwave radiation products include spatial heterogeneity, input parameter errors, cloud influence and low spatial resolution of radiation products and so on.