Lake mixing regimes not only provide ecosystem services to the lacustrine status but also interact with the regional climate. However, field observational data on the vertical water temperature of lakes is scarce on the Tibetan Plateau, which inhabits a clear understanding of the actual thermal structures of lakes in high cold regions. Based on the water temperature profiles and meteorological data of La'ang Co in the Tibetan Plateau, variations of thermal stratification and mixed layer depth in the hourly and daily scale were investigated. La'ang Co is a cold polymictic lake, and the lake surface temperature peaks in August. The water temperature in the lake centre and along the shoreline exist spactial variability, although both exhibited similar seasonal fluctuations. The surface temperature decreases slowly and stably, which lags behind the air temperature about a month. Furthermore, the variation of the mixed depths in La'ang Co is formed, and solar radiation and wind are key external factors in regulating the mixed layer depths. A better understanding of the mechanism of the lake stratification on the Tibetan Plateau can improve the performance of lake simulation and facilitate the development of lake parameterization.