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• The WetlandC model was developed and combined with the TEM model. • Modelled soil organic carbon density in the alpine wetlands on the Tibetan Plateau. • Precipitation dominated the spatial heterogeneity in soil organic carbon density. • Soil organic carbon density showed widespread increases from 2000 to 2018. • Changes of soil organic carbon stock in alpine wetlands contributed a carbon sink. The Tibetan Plateau has been undergoing profound warming and slight wetting over recent decades, which have contradictory effects on soil organic carbon (SOC) by enhancing plant growth and thereafter carbon input into the soil and increasing the soil organic carbon decomposition rate. In this study, we developed a SOC model (WetlandC model) for wetlands, considering the process of litterfall decomposition and parameterizing the effect of grazing on SOC accumulation. We also established a modelling framework to combine the WetlandC model with the terrestrial ecosystem model (TEM model) to simulate the changes in SOC in the alpine wetlands on the Tibetan Plateau from 2000 to 2018. The results showed that spatially, the soil organic carbon density (SOCD) of alpine wetlands was higher in the southeast and lower in the northwest, ranging from 1358.22 to 22571.81 g C m−2. Precipitation played an important role in regulating the spatial heterogeneity of SOCD. The temporal trends of SOCD varied from −55.84 to 407.59 g C m−2 yr−1, and 97.98% of the wetland area accumulated SOC. Temperature, precipitation and livestock carrying, as the top influencing factors of the temporal trend of SOCD, accounted for 35.1%, 34.5% and 30.4% of the area in the alpine wetlands, respectively. The 0–30 cm SOC stock of the alpine wetlands on the Tibetan Plateau increased from 518.06 Tg C in 2000 to 607.67 Tg C in 2018. The surface soil of alpine wetlands acted as a carbon sink of 4.98 Tg C yr−1. Our results indicated that in the context of climate change, additional soil carbon sequestration in the alpine wetlands was facilitated by enhanced plant growth, although grazing consumed the aboveground biomass. Future climate warming and wetting are likely to benefit SOC accumulation in the alpine wetlands on the Tibetan Plateau if overgrazing does not occur. [ABSTRACT FROM AUTHOR]