Abstract: Reservoir tributaries are important areas for the production and release of greenhouse gas (GHG) due to their special hydrological conditions and biogeochemical processes. In this study, the spatial and temporal distribution characteristics and formation mechanisms of CH?, CO? and N?O were systematically investigated by high-resolution vertical sampling in Yongping River, a tributary of Xiaowan Reservoir of Lancang River. The results showed that significant thermodynamic stratification existed in the study area, and the stability of stratification was significantly stronger in the dry water period (IC = 3.87) than in the abundant water period (IC = 5.72). The stratification structure of the water column affected the characteristics of the vertical distribution of GHG, and the stable stratification led to a CH? concentration of 2.085 μmol/L in the bottom layer, which was much larger than that in the surface layer; CO? appeared to have a maximum value in the thermocline (121.37 μmol/L); and N?O concentration was elevated at the sediment-water interface. Apparent oxygen consumption was significantly and positively correlated with ΔCO? (R2 = 0.46 during the dry period and R2 = 0.15 during the abundant period), suggesting that organic matter degradation is an important source of CO? production. Spatially, the mean value of CO? equivalent in the river-phase section was 641.31 mg CO?eq/m2/d, and its emission gradually decreased as the point was close to the reservoir area; the transition section was the hotspot of GHG emission, and its mean value of CO? equivalent amounted to 764.79 mg CO?eq/m2/d; and the lake-phase section had the smallest CO? equivalent value of 434.49 CO?eq/m2/d. Temporally, the total emission of GHG was 808.79 mg CO? equivalent in the dry season. The total GHG emission of 808.64 mg CO?eq/m2/d was higher than that of 440.64 mg CO?eq/m2/d during the abundant water period, and this difference was more prominent in the lake-phase section and the transition section, while it was not obvious in the river-phase section. Overall, the construction of the reservoir made the transitional section of the tributary become a hotspot of GHG emission, and caused obvious seasonal differences between the transitional section and the lake-phase section. In addition, the tributary GHG emission (624.64 mg CO2eq/m2/d) was larger than that of the main reservoir (337.06 mg CO2eq/m2/d). Therefore, the contribution of tributaries to reservoir GHG emissions cannot be ignored. This study preliminarily elucidated the vertical characteristics and formation mechanism of the GHG distribution in the reservoir tributaries, and suggested that the transition section should be the key monitoring area, and the research results provide important references for the assessment of GHG emissions from the reservoir tributaries and the formulation of emission reduction strategies.