Permafrost-affected peatlands on the central Qinghai-Tibet Plateau (QTP) store globally significant amounts of carbon but face climate-induced hydrological changes. The mechanisms enabling slope peatlands to sustain waterlogging under low net precipitation (precipitation minus evapotranspiration) remain uncertain. We combined field measurements (drone-based topography, peat cores and porewater delta 18O) with a modified DigiBog_Boreal model to elucidate water balance in Chadam peatland, a representative permafrost slope peatland on the central QTP. Laboratory analyses reveal that Chadam peatland is characterized by exceptionally high dry bulk density (0.48 +/- 0.21 g cm-3 (n = 8), compared to 0.12 +/- 0.09 g cm-3 in northern peatlands (n = 1,318)) and low horizontal hydraulic conductivity (K h), indicating distinct hydraulic properties. Model simulations parameterized with local steep slopes (5 degrees), low net precipitation (mean 165 mm yr-1), and site-specific peat properties demonstrated that only the low K h (initial 0.01 cm s-1; as measured in this study) simulation scenario can sustain millennial-scale waterlogging. These conditions facilitate continued peat accumulation, matching observed peat thickness. In contrast, high-conductivity scenarios (initial K h = 0.15 cm s-1 and 3 cm s-1), due to their low water retention capacity, fail to maintain elevated water tables both during peat initiation and over subsequent centuries (300-600 years) under identical climatic and terrain conditions. Aligning with model results, stable isotope (delta 2H and delta 18O) profiles corroborate strong evaporation and prolonged subsurface water residence times. These findings indicate that peatland hydrological stability depends on either low K h, sufficient net precipitation, or gentle topography to counteract destabilizing influences. This study identifies peat hydraulic conductivity as the primary control on hydrological stability in semi-arid, high-elevation permafrost peatlands, providing new insights into QTP peatland resilience under warming conditions.