Fluctuations of water level in Hung-tse Lake are induced by the regional increasingirrigation demands and normalized water transfer with the operation of the SouthtoNorth water diversion project, with consequences on soil moisture and wetland vegetation. A poplar plantation in Hung-tse Lake wetland was selected to investigate the dynamics of CO₂ flux and its influencing factors during the growing season (April to September) using the system of eddy covariance in 2016. The results showed that a typical daily and monthly variation characterized the changes of net ecosystem CO₂ exchange (NEE) and environmental factors across the whole growing season. The monthly meanNEE showed a general “U” type curve with1758.10 g CO₂·m^-2 of accumulatedNEE, implying that the plantation was a carbon sink in the growing season. There was ahyperbolic relationship between the photosynthetically active radiation (PAR) and daytime netecosystem CO₂ exchange (NEE_d).PAR accounted for 34.3%-75.5% of the variation ofNEE_d in the growing season. A significant exponential function relationship between soil temperature (T_s) at 5 cm depth and nighttime net ecosystem CO₂ exchange (NEE_n) was found, andT_s explained38.9%-55.2% of the variation inNEE_n. Soil water content, net radiation (R_n) and vaporpressure deficit mainly dominated the daily variation of CO₂ flux, while the monthly variation of CO₂ flux was mainly affected by rainfall and soil moisture. Therefore, changes of soil moisture in the wetlands, affected by the fluctuations of water level in Hung-tse Lake, would potentiallyaffect ecosystem carbon sequestration.