Thermal stratification and circulation in a lake affected the deep lake water ecosystem significantly. Lake thermal processes change obviously in the realm of the global warming. We conducted an investigation in 60 sites from July 2014 to May 2015 in Qiandao Lake (or Xinanjiang Reservoir build in 1959. Qiandao means thousand isles in Chinese. In maximum, area 580km~2, volume 178.4*10~8 m~3, depth 108m (average depth 31m)), Zhejiang, East China, to analyze the distribution of water temperature and thermal stratification. Relationships of water temperature, transparency, and depth to the thermocline depth were regressed. Thermal stratification parameters based on the in-situ lake-wise physical and chemical data. Results show that thermal stratification appeared in spring and summer, to which water temperature was the main controller, followed by transparency and depth. The depth, thickness, and strength of thermocline increased from upstream to downstream, especially in July than in May. In the summer, the thermal stratification was stable, and the thermocline depth showed significantly negative relation with the surface temperature, and positive with transparency and depth.