Water samples of Lake Poyang and its five inflow river were collected in typical hydrological seasons from 2019 to 2020, to reveal the spatial variations of dissolved inorganic carbon (DIC) and its isotopic composition, as well as to estimate the carbon flux and the sources of the three main carbons in Lake Poyang. The results indicated that DIC concentrations in wet seasons were higher than those in dry seasons whereas both higher δ¹³C_DIC and the δ¹³C_DIC in dry seasons. The DIC flux in the surface water of Lake Poyang ranged from 266.1 to 268.4 t/month (the average was 267.3 t/month) in wet seasons, and ranged from 2.88 to 5.94 t/month (the average was 4.41 t/month) in dry seasons, exhibiting a difference nearly 60.6 times in different seasons. The contribution of gas CO₂ exchange between lake water and atmosphere to the DIC flux in Lake Poyang was the largest, accounting for 73.07%, followed by river input with 15.53%. The weathering of carbonate rock by H₂CO₃ was the dominating reaction controlling both DIC sources and δ¹³C_DIC compositions, and the variation of seasonal rainfall runoff was the main controlling factor of the contribution of different carbon sources to the carbon flux in Lake Poyang.