Inter-tidal salt marsh plays a great important role in the global carbon cycle as a considerable potential capacity of the carbon sink. Compared to other types of wetlands, the inter-tidal salt marsh has a unique biogeochemical process under the combined action between freshwater and seawater. Therefore, there is considerable variability and uncertainty in its net ecosystem CO₂ exchange (NEE). However, few studies provide insights regarding the variability of NEE and its controlling factors in an inter-tidal salt marsh. Using the Eddy Covariance (EC) technique, we analyzed temporal variation in NEE and determined its control mechanisms coupled with meteorological and tidal inundation variables during the growing season (from April to October) of 2012 in an inter-tidal salt marsh in the Yellow River Estuary. The results showed that it was net CO₂ absorption in the daytime and net CO₂ release in the nighttime on a diurnal scale. The daily average NEE during the growing season was-0.38 g CO₂ m^-2 d^-1, with a maximum daily CO₂ uptake rate of-3.13 g CO₂m^-2 d^-1 (June 27) and a maximum release rate of 1.47 g CO₂ m^-2 d^-1 (August 12). The monthly average NEE increased rapidly from May, and peaked in June, then decreasing gradually from July. The maximum monthly ecosystem respiration (R_eco) was 15.16 g C/m² in June when the hightest soil temperature was 27.5 ℃. The monthly gross primary productivity (GPP) reached its peaking value (25.07 g C/m²) in July. During the growing season, NEE was mainly dominated by photosynthetic active radiation (PAR), soil temperature (T_s), soil water content (SWC) and tidal inundation. There was a rectangular hyperbolic relationship between the daytime net ecosystem CO₂ exchange (NEE_daytime) and PAR. The maximum ecosystem apparent quantum yield (α) and maximum photosynthesis rate (NEE_sat) appeared in June ((0.0086±0.0019) μmol CO₂ μmol^-1 photons) and in May ((4.79±1.52) μmol CO₂ m^-2 s^-1), respectively. In addition, NEE_daytime also was positively correlated with T_s and SWC. During the growing season, NEE_daytime had an exponential relationship with T_s. The mean value of Q₁₀ was 1.33, and it was positively related to SWC. During the typical sunny day of June 19 to June 25, tidal inundation enhanced daytime net absorption of CO₂ and nighttime CO₂ release. As a result, tidal inundation increased the net ecosystem CO₂ absorption by an average of 0.76 g CO₂ m^-2 d^-1. During the growing season, the inter-tidal salt marsh was an obvious CO₂ sink (22.28 g C/m²), with a cumulative emission of 96.28 g C/m² and a cumulative aborption of 118.34 g C/m².