<p>Understanding the decadal variability of the East Asian monsoon over the past millennium is essential for improving projections of its future change. Recent advances in paleoclimate reconstructions and climate modeling have expanded our knowledge, yet most model-proxy comparisons remain focused on precipitation or dryness indices, making it difficult to directly relate model outputs to oxygen isotope (<i>δ</i>¹⁸O) records. Here, we use simulations from the isotope-enabled Community Earth System Model-Last Millennium Ensemble (iCESM-LME) to investigate changes in precipitation <i>δ</i>¹⁸O (<i>δ</i>¹⁸O_p) and compare them with proxy records. We find that <i>δ</i>¹⁸O_p exhibits a pronounced quasi-11-year cycle at decadal scales across the monsoon region. Unlike the precipitation field, which shows a tripolar wet-dry-wet spatial pattern, <i>δ</i>¹⁸O_p displays a more uniform regional signal. By comparing the solar forcing experiment with the control experiment, we identify solar activity as a key driver of this decadal <i>δ</i>¹⁸O_p variability. To explore the mechanism, we conduct water-tagging experiments under solar maximum and minimum phases. The results show that increased solar irradiance modulates a La Niña-like sea surface temperature pattern, which enhances the Walker Circulation. This leads to enhanced upstream convection over the Maritime Continent and a greater contribution of moisture from the equatorial Pacific, resulting in lower <i>δ</i>¹⁸O_p values across the monsoon region. Our findings suggest that the quasi-11-year <i>δ</i>¹⁸O_p cycle primarily reflects shifts in moisture sources driven by solar activity. This study enhances model–proxy comparability and deepens our understanding of how <i>δ</i>¹⁸O_p captures decadal-scale dynamics of the East Asian monsoon.</p>