Studies have suggested that, globally, lake and reservoir systems constitute large carbon (C) sources. However, the drawdown zone where certain lake areas subject to water level fluctuations temporarily dry out, exposing sediment to the atmosphere, needs to be considered in lake-based C sink/source quantitative assessments. This study incorporates the drawdown zone into a lake-based quantitative approach to investigate C source/sink and C pool pattern stability in Poyang Lake and the surrounding wetland ecosystem, a typical floodplain-lake system case. By combining field sampling and remote sensing methods we found that from flooding to non-flooding periods, the C storage of Poyang Lake increased, leading to a steady C source. When accounting for the drawdown zone, however, the whole floodplain-lake system shifted from a C source to a C sink. Temperature drives changes in wetland C emissions and vegetation C production by affecting microbial organic carbon (OC) decomposition processes, to regulate this unsteady C source/sink lake system transformation processes (R²=0.97). Rainfall-driven water level and area changes can also influence C storage of the whole floodplain-lake system (R²=0.86). It is therefore necessary to integrate the drawdown zone into lake system qualification processes to ensure steady C source and sink states.