In order to investigate the C〇2 in water of the backwater area of the Wanfenghu Reservoir in summer, and whether thisarea is a resource or a sink of the CO 2 in air, the partial pressure of CO2 in the backwater area was detected by an underwayobservation system in September 2016. The investigation employed Ultrameter (6P) to detect the water quality parameters in thebackwater area, the other parameters were detected in the laboratory. The results show that the /?C〇2 in surface water of thebackwater area is 150.58 -220.05Pa. According to data on different soundings, the /?C0 2 in surface water is the lowest and itincreases steadily with the depth of water. The /?C〇2 goes up after a slight drop at a depth of 50m and reaches a peak at a depthof 80m. The dissolved quantity of CO2 in the backwater area in summer is supersaturated, different from the undersaturation inmost lakes in Yun -Gui Plateau and reservoirs. This phenomenon is relevant to the surroundings, the features of water of thebackwater area as well as a large number of sediments and resolvent in it. In reference to interaction formula, the diffusion flux ofCO2 is 27.4—44.2mmol *4 (m2*d)_1 and the mean value is 37.1mmol ? (m2*d)_1. The water of the backwater area represents as theresource of the C0 2 in the air.