The Advanced Geostationary Radiation Imager (AGRI) is one of the main payloads of Fengyun-4A (FY-4A). In order to satisfy requirements of high-accuracy quantitative application of AGRI long-wave infrared (LWIR) remote sensing data, a function model of scan mirror thermal radiation changing with mechanical rotation angle is constructed, and a correction algorithm for remote sensing data of on-board blackbody and earth scene observation is presented. Based on the on-orbit data of FY-4A AGRI, the dependence of the model parameters on the scan mirror temperature is analyzed, and the improvement effect of the correction algorithm on the calibration accuracy of LWIR is studied and evaluated. After thermal radiation correction of scan mirror in the stationary period of mirror temperature field, the calibration deviation of B11(8.0~9.0μm), B12(10.3~11.3μm), B13(11.5~12.5μm) can be improved significantly by -2.81K~+1.06K, -0.60K~+0.19K, -0.68K~+0.24K respectively. The result of inter-calibration validation with IASI show that the improved on-orbit calibration brightness temperature bias is better than 0.5K@290K.