Millennial-scale climate variability (MCV) in the Asian summer monsoon (ASM) plays a crucial role in elucidating short-term monsoon dynamics and in improving projections of future Asian hydroclimate change. However, significant divergences remain in our understanding of how high- and low-latitude climate systems couple to drive and modulate MCV in ASM precipitation. These discrepancies primarily arise from a limited understanding of the spatial heterogeneity in precipitation MCV in marginal ASM regions. We present a new high-resolution (similar to 280-year) loess leaf-wax hydrogen isotope (delta Dwax) record from the central Chinese Loess Plateau (CLP) spanning the last glacial period. The record reconstructs regional precipitation isotopic variability on precessional and millennial timescales. The strong correspondence between the CLP delta Dwax record and 65 degrees N summer insolation demonstrates that orbital-scale variations in precipitation delta D across ASM marginal regions primarily reflect high-latitude insolation forcing. On millennial timescales, although the MCV of Xifeng (XF) precipitation delta D exhibits characteristics modulated by the precession periodicity, its exceptionally low mean absolute amplitude indicates that this modulating effect was significantly attenuated in this region during the last glacial period. Synthesis of precipitation isotope records across the monsoon region reveals complex spatial heterogeneity in the amplitude of MCV during the last glacial period. The Indian monsoon-affected region exhibits greater MCV overall than the East Asian monsoon-dominated region. Although both monsoon systems exhibit a trend of strengthening amplitude along their respective moisture transport pathways, MCV amplitudes decrease in the transitional zone. This pattern suggests that these regions were situated beyond the ASM marginal zone during the glacial period. Our findings further underscore that the pathways and strengths of moisture transport critically govern the spatial heterogeneity of monsoon precipitation MCV, revealing a glacial-state hydroclimate pattern where a region's response amplitude is dictated by its relative position on the moisture pathway.