Based on the infrared radiative transfer calculations for 446183 atmospheric profiles, the downward longwave radiation flux at surface and channel brightness temperatures for the FY-4 Imager were simulated. From a regression analysis of the simulated data, a retrieval model relating the flux with channel temperatures and atmospheric precipitable water was established. The model was then applied to Meteosat-8 SEVIRI (Spring Enhanced Visible and Infared Imager) data, adopted as a proxy of the FY-4 Imager, and the downward longwave flux was estimated over an area of (45°S-45°N, 45°W-45°E), at 0000 UTC, 0600 UTC, 1200 UTC, and 1800 UTC Aug 1 2006. The fluxes were then compared with those calculated using empirical equations on ECMWF data. The RMS errors were 12.1, 13.0, 20.7, 12.5 W/m², respectively. Compared with the RMS 13.7 W/m² value from GOES-R, the estimates from the model developed in this study would satisfy future climate research requirements.