Water stable isotopologues, particularly H₂¹⁸O and HD¹⁶O, are valuable tracers of physical and dynamical processes within the hydrological cycle. These isotopologues have been widely incorporated into isotope-enabled atmospheric general circulation models to constrain moisture sources and transport pathways. However, comprehensive evaluations of such models over the Tibetan Plateau (TP), a region whose complex orography substantially modulates South Asian monsoon dynamics, remain scarce. Here, we systematically evaluate simulations of water vapor isotopic composition (δ¹⁸O_v) from the ECHAM6-wiso model against daily in-situ observations from four high‐altitude stations (Kathmandu, Lulang, Namco, and Muztag) on the Tibetan Plateau, spanning January 2020 to November 2021. The model successfully reproduces the spatial distribution and seasonal cycle of δ¹⁸O_v; however, probability density functions reveal a systematic underestimation of isotopic depletion. Through a multi‐scale temporal decomposition of the daily δ¹⁸O_v time series, we attribute over 50% of the simulation error to deficiencies in representing large-scale atmospheric circulations (periods ≥ 30 days), while the remaining error is linked to synoptic-scale processes (3–7 days) associated with fractionation, including cloud microphysics, post‐condensation effects, and surface evaporation. The model's inability to accurately simulate terrain-induced atmospheric moisture blocking over the TP results in bias in atmospheric circulation variations, thereby amplifying the contribution of circulation-related processes to the overall error. These findings underscore the significance of atmospheric circulation in water vapor isotopic simulations and highlight the value of high‐resolution water vapor isotopic datasets for improving our understanding of moisture source attribution and water‐cycle dynamics in regions of complex topography.