In this study, the specific methods of estimating surface longwave radiation fluxes under allsky condition are elucidated. An inverse model for cloudysky surface downward longwave radiation (DLR) flux retrieval is developed based on infrared radiative transfer calculation, statistical regression analysis of simulated data, and atmospheric profiles. After that, we combine the inverse model and GFS data to calculate FY4A cleansky and cloudysky DLR values. Then, on the basis of the Aqua/CERES global DLR product, it validates the corresponding product of FY4A during day or night time. The final results show that RMSE=20.52 W·m-2, R=0.9481， BIAS=3.3 W·m-2 (nighttime); RMSE=25.58 W·m-2，R=0.9096， BIAS=5.4 W·m-2 (daytime). Besides, we also successfully calculate the cloudysky upwelling longwave radiation (ULR) product at surface, which is combined with the operational FY4A clearsky ULR product to make a joint allsky product. The validations of FY4A allsky ULR show that RMSE=10.97 W·m-2，R=0.9762，BIAS=-3.3 W·m-2 (nighttime); RMSE=19.97 W·m-2, R=0.9283，BIAS=5.0 W·m-2 (daytime). The validation results mentionedabove indicate the retrieval methods developed in this research are feasible and highquality, which could be used for retrieving new operational DLR/ULR products for FY4 series satellites in future.