Revealing the subtle variations in thermal structure in a reservoir is crucial for accurately understanding the interaction between water and carbon cycle. This study provides a detailed description of the thermal structure of Dalongdong reservoir, a representative subtropical stratified reservoir in Guangxi, China. It unveils the three stable state changes in water, including mixing, microstratification, and stratification, along with their influencing factors. The study emphasizes air temperature (AT) as the primary factor influencing water temperature (WT) stability, followed by water level. Cross-correlation analysis shows that adjacent water temperatures experience a prolonged response hysteresis during the stratification period, while the mixing period exhibits a shorter hysteresis. Wavelet analysis revealed that AT and surface-layer WT exhibited a dominant oscillation period of 16 - 32 hours. The periodicity of other water layers is not obvious. This framework can help to enhance our understanding of material cycles and the protection the water environment.