There is no 1 km spatial resolution all-weather land surface temperature (LST) product based on domestic satellites in China. FY-3D satellite provides the clear sky LST products with 1 km resolution from the medium resolution spectral imager (MERSI) Ⅱ and the all-weather LST products with 25 km resolution from the microwave radiation imager (MWRI). Therefore, the integration research of all-weather 1 km resolution LST can be carried out by combining their advantages. By using geographical weighted regression (GWR) method, this study selects altitude, FY-3D normalized difference vegetation index and normalized difference building index to establish LST downscaling regression model in order to downscale FY-3D/MWRI 25 km LST to 1 km, and integrates them with MERSI 1 km LST. For the MWRI track gaps, 1 km LST of cloud covered pixels fused in the previous one day and the next day can be used to supplement, which is close to the all-weather 1 km LST. Based on the above fusion algorithms, multiple Chinese FY-3D/MERSI and MWRI LST products on typical dates from the official website are selected for fusion test, and the existing all-weather 1 km LST products (TPDC LST) were used to evaluate the results of FY-3D 1 km LST fusion products. The results show that the LST downscaling method based on GWR method can effectively eliminate the “patches” effect and low local temperature in traditional microwave downscaling methods based on the combination of altitude with mixed pixel decomposition. The rate of FY-3D 1 km LST can be increased from 22.4%-36.9% before fusion to 69.3%-80.7% after fusion. The spatial correlation between the fusion product and TPDC LST is 0.503-0.787, and the RMSE is 3.6-5.8 K with 2.6-4.9 K in clear sky and 4.1-6.1 K in cloudy sky. The analysis also shows that the current FY-3D/MERSI LST and MWRI LST products from the official website have problems such as obvious lack of value and low accuracy, suggesting that they have a great potential to be improved. This is conducive to further improving the quality of FY-3D LST fusion.