The phenomenon of sudden water quality degradation caused by thermal stratification extinction in reservoirs has attracted widespread attention. The reservoirs are mostly warm lakes in southern China that are mixed once a year, resulting in cyclical water quality degradation. However, research on high-frequency monitoring of thermal stratification decline is relatively limited. To investigate the spatial and temporal variation patterns and driving factors of water mixing processes during the thermal stratification extinction period of reservoirs in southern China, taking Tianbao Reservoir in Nanning City, Guangxi as an example. The physical and chemical indices of multi-point water bodies in the reservoir were monitored in situ during the winter thermal stratification extinction period (from November 2019 to February 2020), and meteorological and hydrological data were collected using self-designed weather stations. The results show: (1) During thermal stratification extinction, the transition zone can reach a full mixing state in a short time and the anoxic zone disappears synchronously. The transition zone of the reservoir reaches the full mixing state about 40 days earlier than that of the lake. The depth of the mixed layer increased from 6.85 m to 13.65 m, the anoxic zone gradually decreased and the anoxic index (AI) decreased from 0.40 to 0.07 until the anoxic zone disappeared after the water was completely mixed. (2) Meteorological factors are the main drivers of changes in the thermal stratification structure. Temperature (T) and radiation (R) show a significant negative correlation with mixed layer depth (MLD) (R_T=0.93, R_R=0.93, P<0.01). Wind speed shows a significant positive correlation with MLD (R_WS=0.73, P<0.01). Temperature change is the key factor for the failure of the thermal stratification structure. (3) The physical and chemical indicators of the water body during the period of thermal stratification extinction were significantly influenced by the degree of mixing. Water temperature shows a significant negative correlation with MLD (R=0.87, P<0.01). DO, pH and ORP show a significant positive correlation with MLD (R_DO=0.75, R_pH=0.76, R_ORP=0.75, P<0.01). Thermal stratification extinction provides the environmental conditions for sudden water quality deterioration. It is necessary to pay close attention to the influence of changes in physical and chemical indices on the water quality of the whole reservoir.