Abstract East Asian Summer Monsoon (EASM) varied at orbital and millennial timescales during the Last Deglaciation (∼20–11 ka). The uniform δ18O response in East China is often linked to EASM circulation that drives a precipitation dipole in North and South China on orbital scale. Yet the relationship between stalagmite oxygen isotopes (δ18Oc) and regional rainfall remains ambiguous at millennial timescales. Combining proxy records with isotope‐enabled transient climate simulations (iTraCE), we demonstrate a decoupling between changes in δ18Oc and regional precipitation in North China during millennial‐scale events. Although δ18Oc responds uniformly across East China to AMOC changes and is correlated with South China rainfall, North China rainfall is instead largely driven by insolation, with negligible millennial‐scale variability. This reflects that East China δ18Oc is driven by AMOC‐induced shifts in moisture source δ18O. Our findings emphasize that δ18Oc records require forcing‐specific interpretation in East Asia across timescales.