Green house gases emission from artificial reservoir is a hot spot in current researches on global climate change. However,related studies on the different types of reservoirs are still inadequate. In order to understand characteristics of the CO_2 emission from Run-of-the-river reservoir in southern China,a seasonal investigation on the Wan'an Reservoir in Jiangxi Province was carried out from 2014 to 2015. This study has used an online system(HydroCTM /CO_2) to measure pCO_2 in water,water quality parameters,and meteorological parameters at various sampling locations in this reservoir. The results show that due to the short time period of water retention,there is no seasonal thermal and chemical stratification in the water column of the Wan'an Reservoir. However,there are obvious seasonal differences of water quality parameters rather than the spatial heterogeneity; with respective average values of 46.00,44.58,57.63,and 17.09 mmol·m~(-2)·d~(-1) for the CO_2 emission fluxes from water-air interface of the Wan'an Reservoir in Spring,Summer,Autumn,and Winter,respectively. Based on this study and other reports,a model for retention time of CO_2 emission from the reservoir has been proposed.