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• The Dongfanghong peatland originated in 1011 cal. CE post Millennium Eruption, expanding laterally through paludification. • The carbon accumulation averaged 118 g C/m2/yr, ranging from 38 to 374 g C/m2/yr. • Climate mainly governed multi-centennial carbon accumulation rates. • Hydrological conditions regulated both lateral expansion and carbon accumulation. Peatlands are terrestrial ecosystems with high carbon sequestration efficiency, providing critical feedback on climate change. It is crucial to understand how peatland development and carbon accumulation respond to various factors to reveal the future direction of carbon reservoirs under changing climatic conditions. To assess the impact of climate and hydrological conditions on the development and carbon accumulation of peatlands, 15 peat core samples were analyzed to examine the processes of lateral expansion in the Dongfanghong (DFH) peatland, a minerotrophic fen located in the Changbai Mountains of northeast China. Accelerator Mass Spectrometry 14C, total organic carbon, dry bulk density, testate amoebae, δ13C of α-cellulose in Carex spp. and plant macrofossil analyses were conducted on a high-resolution peat core. The proxies were used to calculate carbon accumulation rate (CAR) and reconstruct water table depth as well as surface moisture and vegetation development of the DFH peatland. Our findings indicated that the DFH peatland was initiated at 1011 cal. CE and experienced paludification during the last millennium. The expansion of the DFH peatland was primarily determined by hydrological conditions rather than climatic factors such as total solar irradiance and regional temperature. The rapid expansion between 1500 and 1700 cal. CE was likely resulted from relatively low precipitation and a decrease in water table/surface moisture. CAR generally correlated with total solar irradiance, regional temperature, and precipitation, suggesting that climate may be crucial for CAR in DFH peatlands over multi-centennial timescales. However, the increased CAR under relatively dry conditions suggested that the CAR were regulated by hydrological conditions over a centennial timescale. This study highlighted the important roles of climate and hydrological conditions in the lateral expansion and CAR of the peatland in minerotrophic fen and that there may be a threshold for hydrology during the process of peatland development and carbon accumulation. [ABSTRACT FROM AUTHOR]