Abstract: The carbon role of water storage and power generation reservoirs has received continuous attention in recent years. In order to investigate the effect of dam construction on reservoir greenhouse gases (GHG) emissions, this paper uses the reservoir net greenhouse gas flux assessment model (G-res Tool) to calculate the GHG emissions of the Dahua Reservoir, the first reservoir built on the upper karst region of the Pearl River. The analysis results shows that the total annual average GHG emissions (CO2 and CH4) of the reservoir increase by 2.5 times compared with that of the reservoir before impoundment as the water area increases. However, due to the geographical and anthropogenic constraints, the average annual greenhouse gas emission flux from the reservoir after impoundment is 216 gCO2e/(m2·a), which is significantly lower than the average emission of hydroelectric power generation under the same latitude and climate conditions, and significantly lower than the emission of thermal power generation of the same scale. The research results can provide effective technical support for the scientific assessment of GHG carbon attributes of other hydroelectric projects in the same type of region.