<p>Deciphering speleothem oxygen isotope records in the East Asian monsoon region is critical for understanding hydroclimate variability, yet the role of upstream effects remains poorly quantified. Here, we combine an isotope-enabled atmospheric model with water-tagging capability and a backtracking algorithm to develop a physically based precipitation-weighted Upstream Accumulated Precipitation (weighted-UAP) index that quantifies the impact of upstream effects on East Asian precipitation δ¹⁸O. Our framework reveals that Indo-South Pacific moisture, despite contributing only a small fraction of total precipitation, accounts for up to 46% of the interannual variance in East Asian precipitation δ¹⁸O, as it acquires a strong depleted isotope signal through extensive fractionation during transport. Specifically, the negative East Asian precipitation δ¹⁸O anomaly is primarily driven by enhanced Tropical Southern Pacific Ocean (TSPO) moisture contribution, facilitated by a La Niña-like South Pacific anticyclone. Concurrently, the Indian Ocean (IO) moisture amplifies this isotopic depletion through a coupled increase in both enroute precipitation and moisture source contributions. This framework further elucidates the non-stationary relationship between ENSO and East Asian precipitation δ¹⁸O arises from asynchronous changes in ENSO and the Indian Summer Monsoon. Our findings provide a novel perspective for interpreting speleothem δ¹⁸O and associated hydroclimate changes.</p>