Based on the CN05.1 monthly mean temperature, the ERA5 (ECMWF Reanalysis v5) reanalysis, and the monthly mean SST (sea surface temperature) of ERSSTv5 (Extended Reconstructed Sea Surface Temperature version 5) from 1979 to 2021, the high temperatures over the regions south of the Yangtze River (RSYR) in September 2021 and their linkage with the SST anomalies (SSTAs) in the southern Indian Ocean (SIO) are analyzed. The results are as follows. In September 2021, abnormally high temperatures are found over the RSYR in eastern China. The high temperature anomaly over RSYR (24.2°-30.8°N, 108.5°-117.5°E) reaches about 3.33 ℃. After removing the long-term trends, it is still about 2.46 ℃, which is the highest in September during the past 4 decades. Further analysis shows that there is a linkage between the abnormally high temperatures over RSYR and the cooling in the SIO during September 2021. The negative SSTAs in SIO are accompanied by an anticyclonic circulation anomaly in the lower troposphere. Simultaneously, anomalous low-level divergence and high-level convergence are observed over the majority of this sea area. The SIO cooling also causes anomalous low-level convergence and anomalous high-level divergence over the Maritime Continent (MC), which is controlled by anomalous ascent motion. These large-scale circulation anomalies can strengthen the local Hadley circulation over East Asia and abnormal downdraft over RSYR, which causes high temperature weather there. Furthermore, the positive SSTAs in the tropical southeast Indian Ocean is accompanied by the non-adiabatic heating, the Matsuno-Gill response of the lower level induces the easterly anomalies prevailing from the MC to the tropical western Pacific, and the western Pacific subtropical high (WPSH) lies to the west of its normal and is slightly intensified, which is conducive to maintain the anomalous descent motion over RSYR. Large-scale heating budget analysis indicates that the negative SIO SSTAs can cause the negative non-adiabatic heating anomaly over RSYR by regulating the non-adiabatic heating anomaly over the MC region. The positive vertical temperature advection anomaly due to the descent motion contributes to the high temperatures over RSYR in September 2021.