Greenland Interstadial-21 (GIS 21), with a centennial-scale precursor event and a “rebound-type” event, was one of the longest warm millennial-scale events recorded in Greenland ice cores during the last glacial period. Precise determination of its timing and duration can improve our understanding of regional/global climate correlations over Marine Isotope Stages (MIS) 5b/5a and 5a/4. Here, a precisely 230Th-dated decadally-resolved stalagmite δ18O record from Xianyun Cave, Fujian Province, southeastern China, is utilized to reconstruct the sub-millennial-scale changes in East Asian summer monsoon (EASM) from 87.6 to 76.3 kyr BP (before 1950 CE), covering Chinese Interstadial 21 (CIS 21) event. A comparison of spatially-separated Chinese speleothem records during GIS 21 shows a decrease in speleothem δ18O values from southeastern China to northwestern China. These δ18O spatial gradients could be attributed to the “rainout effect” and the amplification mechanism of the seasonal variations of precipitation δ18O. Moreover, Xianyun δ18O record captures the precursor event centered at 85.3 ± 0.3 kyr BP concurrent with the corresponding event in ice cores, revealing an atmospheric teleconnection between high and low latitudes. Unlike gradual cooling at the Greenland and Antarctic ice core records during mid-GIS 21, the Xianyun record reveals a relatively strong monsoon phase. We speculate that the cooling of the Southern Hemisphere (SH) may influence EASM activity through the intensification of cross-equatorial flow and weaken the impact of Northern Hemisphere (NH) high latitude climate, thus pointing to the role of SH climate on EASM on millennial-scale.