Peatlands are carbon-rich wetlands recognized for providing important ecosystem services. In the hyperhumid southeastern Tibetan Plateau, subalpine peatlands develop within glacier valleys of the Gaoligong Mountains. Here, based on detailed paleoecological records from multiple cores collected at representative sites, we investigate their long-term development and carbon accumulation histories to understand the processes and mechanisms governing peatland dynamics under conditions of excessive water and their sensitivity to climate change. Regional peatlands initiated their development during 4700-3900 cal. yr BP and directly above sloping, rocky substrates, likely triggered by decreased precipitation that weakened hydrological disturbances and allowed for the preservation of initial peat. They underwent fen-to-bog transitions at multiple timings including 1200-1000 cal. yr BP, 350-150 cal. yr BP, and recent decades, when climate shifted from drier to wetter conditions. The prior drying shaped denser (higher bulk density) and root-rich soils that set suitable hydrological environments for Sphagnum colonization, and the subsequent increased precipitation further enhanced ombrotrophy and triggered the shifts. The peatland carbon accumulation rates exhibited spatial-temporal heterogeneity and were controlled by localized hydrological disturbances and associated mineral inputs rather than climate or vegetation. Gradual water accumulation and overland flooding increased and decreased carbon accumulation rates, respectively. Their long-term mean rates were only 17 g C/m2/yr, much lower than the world's other peatlands under similar temperatures and likely resulting from sunlight limitation on plant productivity and strong lateral carbon losses. Therefore, peatland landscapes in world's hyperhumid mountainous regions are vulnerable to natural degradation under a wetter future.