Wetlands represent crucial carbon reservoirs within terrestrial ecosystems, functioning as either carbon sources or sinks through complex carbon cycling processes. However, the day-night variations in CO? flux across wetland environments remain inadequately understood, creating uncertainty in accurate assessments of wetland carbon sequestration capacity. This study investigated the temporal dynamics and controlling factors of CO? flux in Poyang Lake wetland during 2021 (a normal flow year), employing eddy covariance measurements with partial correlation and multiple regression analyses during both exposed and inundation periods. The results show that: (1) During the exposed period (January-April and November-December), diurnal CO? flux exhibited characteristic "U"-shaped variations, functioning as a carbon sink during daylight hours and a carbon source at night. Conversely, during the inundation period (May-October), CO? flux remained near zero, though with notable day-night fluctuations emerging during late inundation (September-October). (2) Monthly analyses showed significant day-night CO? flux differences during exposed periods that diminished during inundation, closely corresponding with the transformation between carbon source and sink functions. (3) Annual assessments demonstrated pronounced day-night variations, with average nighttime flux exceeding daytime by 25.5%; this diurnal difference peaked during exposed periods (averaging 10.22 μmol·m?2·s?1). (4) Controlling factors varied by period: during exposed periods, daytime CO2 flux was primarily regulated by incident shortwave radiation and soil moisture, while nighttime CO2 flux was dominated by soil temperature; during inundation, daytime CO2 flux responded mainly to precipitation and soil moisture, while nighttime CO2 flux was jointly influenced by soil moisture, lake water level, and soil temperature. (5) The fundamental mechanisms driving these diurnal differences were period-dependent. During exposed periods, biological processes, including photosynthesis (daytime) and respiration (nighttime), created distinct carbon sink-source dynamics. During inundation, water coverage suppressed both plant and microbial activities, consequently reducing day-night CO2 flux variations. These findings elucidate the diurnal mechanisms governing wetland CO? flux, providing valuable scientific basis for integrated carbon-water resource management and wetland ecological protection.