Three diel field campaigns and one monthly sampling campaign during June 2010-May 2011 were carried out to investigate the CH4 flux across the water-gas interface in Xiangxi Bay of the Three Gorges Reservoir, China. The average CH4 flux was much less than that reported from reservoirs in tropic and temperate regions. The photosynthesis of phytoplankton dominated the diel gas fluxes during alga bloom in spring and summer. The maximum monthly flux occurred in June 2010 and corresponded to the lowest water level. Water temperature, sediment temperature, and TOC did not have significant correlation with the monthly CH4 fluxes. Continuously decreasing hydrostatic pressure and the low water level resulted in more CH4 emission at the sediment-water during the discharging period, and thus increases the CH4 effluxes because the diffusion time through a thin water column is shorter and less CH4 may be oxidized compared with that in a long water column.