In order to explore the temporal and spatial distribution of the partial pressure of carbon dioxide (pCO₂) and its diffusion flux in the surface water of karst wetland, the largest karst wetlands in China, Weining Chaohai, Guizhou Province, was selected as the research object. The surface wetland water of Lake Caohai was systematically collected through the method of grid placement in July (wet season) and December (dry season), respectively, in 2019. The physicochemical parameters and ion composition of the water samples were measured, the pCO₂ of the water was calculated by PHREEQCI, and the diffusion flux of carbon dioxide (CO₂) at the water-air interface was estimated base on the gas diffusion model proposed by Cole. The results show that the variation of pCO₂ in the surface water of Lake Caohai wetland ranges from 0.44 μatm to 645.65 μatm in the wet season, with an average value of (55.94±124.73) μatm, while 35.48 μatm to 707.95 μatm in the dry season, with an average value of (310.46±173.54) μatm. In general, the distribution of pCO₂ was higher in dry season than in wet season, and higher in the eastern lake area and the entrance of river to the lake but undersaturated in the midwest area of the study region, spatially. The diffusion flux of CO₂ in Lake Caohai wetland water ranges from -43.27 mmol/(m²·d) to 27.16 mmol/(m²·d), with an average value of (-34.49±12.93) mmol/(m²·d) in the wet season, while ranges from -33.36 mmol/(m²·d) to 28.15 mmol/(m²·d), with an average value of (-8.02±15.85) mmol/(m²·d) in the dry season. Compared with other karst lakes and reservoirs, Lake Caohai, which is rich in aquatic plants, has relatively low CO₂ diffusion flux during the two extreme hydrological periods, and is generally a sink of atmospheric CO₂.