Deep reservoirs with stable stratification often ‘turn over’ in winter, leading to a decline in overall water quality and affecting the safety of drinking water. To explore the mixing characteristics of water bodies during the thermal stratification period in southern reservoirs, long-term in-situ monitoring of physical and chemical indices of Tianbao Reservoir in Nanning was conducted from November 2019 to February 2020 during the thermal stratification fading in winter. Combined with the vertical diffusion coefficient of water, the variation characteristics of water temperature (T) and dissolved oxygen (DO) in the whole process of thermal stratification of the reservoir were described by stages (thermal stratification, weakening, complete mixing and formation), and the influences of meteorological factors (air temperature, solar radiation, wind speed, etc.) and hydrodynamic factors (vertical diffusion coefficient K_z ) on the physical and chemical indices of water were discussed. The results showed that the temperature stratification and the dissolved oxygen stratification of the reservoirs were out of sync during the extinction of the thermal stratification, and the destruction of the dissolved oxygen stratification lags behind that of the water temperature stratification, and its structural instability was the result of the combined action of air temperature and solar radiation. During this period, the vertical diffusion coefficient of the water body varied from 0.16 to 380.36 m²/d, with an average value of 30.46 m²/d. The main controlling factor of surface K_z change was air temperature change, and surface K_z was more sensitive to air temperature change than T and DO (R_Kz =-0.44, R_T=0.25, R_DO=0.25, P<0.01). The change in bottom K_z was consistent with the change in water mixing, which could more accurately characterize the vertical mixing of water. The vertical distribution of T, DO and lg K_z in the water body was temporally heterogeneous. The lg K_z was positively correlated with T and DO during thermal stratification and semi-mixing (R_T =0.60, R_DO =0.76, P<0.01), while it was negatively correlated with T (R_T=-0.19, P<0.01) and had no correlation with DO (R_DO =0.04, P>0.05) after complete mixing. Thermal stratification extinction provides environmental conditions for water quality deterioration, so we should pay attention to the change of meteorological factors in winter and take precautions against it in reservoir monitoring and management.