In order to understand the influencing factors and the diffusion characteristics of CO₂ partial pressure (p(CO₂)) and CO₂ source/sink mechanism in the Xinfengjiang Reservoir, the physicochemical and biological quality parameters of surface water samples collected in July 2012 and January 2013 were measured, and the value of p(CO₂) in water was calculated as well. The results showed that:In wet season, the p(CO₂) ranged from 16 to 3545 μatm with an average of 999 μatm, which increased from the reservoir head region to the dam; During dry season, p(CO₂) ranged from 399 to 1355 μatm with an average of 756 μatm, which decreased from the reservoir head region to the dam. p(CO₂) was less likely affected by the temperature in both wet and dry season. In wet season, there was no obvious relationship between p(CO₂) and dissolved inorganic carbon (DIC). p(CO₂) was positively correlated with nutrient salts (NO₃^-, DSi) concentrations, but negatively correlated with chlorophyll-a (Chl.a) and dissolved organic carbon (DOC) concentrations; In dry season, owing to the carbonate systems, there was no significant correlation between p(CO₂) and DOC concentration, whereas it was positively associated with NO₃^-, DSi, Chl.a and DIC concentrations. The Xinfengjiang Reservoir was the CO₂ source to the atmosphere with a low flux value.