Reservoirs are pivotal sources of greenhouse gases (GHGs) to the atmosphere that may influence regional climate change. However, the spatial heterogeneity and the vertical characteristics of dissolved GHGs in reservoirs are still poorly understood. This study aimed to reveal the characteristics of greenhouse gases dissolution and emission fluxes in reservoirs during stratification and mixing periods, and to provide important supports to clarify the key processes of GHGs production and emission in reservoirs. We selected Tanghe Reservoir, a large reservoir in Northeast China, and conducted vertical stratified monitoring of dissolved GHGs at different locations (in front of dam, mid, end transects) from July-September and October 2021 (representing the stratification and mixing periods, respectively). The results showed that the CH₄ fluxes ranged from 0.018-0.174 mmol/(m²·d) and all sites were the source of CH₄, the spatial distribution of CH₄ fluxes was end>mid>in front of dam. The CO₂ fluxes ranged from -4.91-58.77 mmol/(m²·d), and all sites were the source of CO₂ except for the eastern end transect during the stratified period, and the contents of spatial distribution of CO₂ fluxes was in front of dam>mid>end. In terms of temporal differences, CH₄ fluxes in the stratified period ((0.071±0.044) mmol/(m²·d)) were higher than in the mixed period ((0.027±0.008) mmol/(m²·d)), and CO₂ fluxes in the stratified period ((5.12±5.50) mmol/(m²·d)) were lower than the mixed period ((42.76±16.72) mmol/(m²·d)), and both water temperature and runoff affected CH₄ and CO₂ fluxes. The vertical distribution characteristics of dissolved GHGs indicated that the hot zone of reservoir CH₄ generation was mainly in front of dam and end transects. The CH₄ was heavily accumulated in the bottom water of the dam front and mid transects during the stratified period due to the persistent anoxia in the hypolimnion and the limitation of the exchange capacity in the water column. The comparison with other reservoirs GHGs fluxes in China revealed that the different periods (stratification and mixing period), spatial variations (in front of dam, mid, and end transects) and reservoir ages were the key factors for accurate assessment of reservoir GHGs emissions.