AbstractThe carbon budget dynamics of peatlands plays a crucial role in both the response of peatland ecosystems to environmental change and the effect of environmental change on peatlands. Studies on northern peatlands have suggested that the reliable assessment of the carbon budget must include the carbon flux at the land‐stream interface. But this argument has not been proved in the Qinghai‐Tibet Plateau, the world's largest alpine system. To address the knowledge gap, in the growing season (May to October) of 2016, we chose Riganqiao peatland, a typical peatland in the Zoige Basin of the Qinghai‐Tibet Plateau which has been intensely drained via ditches since the 1960s. The objective of this study was to measure the organic carbon export from ditches, and to estimate what proportion of the carbon budget can be attributed to this export. Results showed that, during the growing season in 2016, total organic carbon flux in ditches was 2,345 t (8.6 g m−2), of which particulate organic carbon accounted for 12% and dissolved organic carbon for 88%. This flux decreased the carbon budget of Riganqiao peatland by 11.2% (from +76.8 g m−2 to +68.2 g m−2), making it a weakened carbon sink. This result revealed that organic carbon export from ditches plays an important role in the carbon balance in peatland of the Qinghai‐Tibet Plateau.