To better understand the factors influencing the thermal structure of tributaries in the Three Gorges Reservoir(TGR),a well validated three-dimensional hydrodynamic and water temperature model was proposed to simulate the water temperature distribution in the Xiangxi Bay,a representative tributary of TGR.The numerical results show that water temperature stratification seasonally occurred in the Xiangxi Bay,with stable vertical temperature profiles.It is found from the numerical experiments that three key factors are responsible for the formation of water temperature structure:(1)very often,the locations of thermocline are mainly determined by wind speeds,and the higher the wind speed is,the deeper the thermocline is located beneath the water surface,which could be expressed by a fitted exponential function,(2)the thermal structure is affected by static stability of water column,and the thermocline becomes closer to the water surface and its thickness increases with the increase of temperature,(3)due to the effect of the thermal density inflow,the water temperature of the hypolimnion tends to be uniform,however,even under the condition of larger inflow discharge,the influence of the inflow on the epilimnion and the thermocline is not significant.