Yuan-Yang Lake (YYL), a polymictic lake located in an ecological protected area, is observed regular variations in season. Meteorological factors, such as atmospheric temperature, wind speed, precipitation, and solar radiation, are analyzed to be inferred as the principle influence on the stratification phenomena or the mixing regime in YYL. These meteorological factors are recorded for each 10-minute interval from 1st June 2008 to 31st May 2009. Through the statistical analyses, Pearson correlation and Multi-regression, the contributing factors of meteorological variables are determined. According to the observation, the mixing depth can be developed to its maximum in winter. In summer, alternative mixing and stratification quickly occurs, because of the multiple changes of strong sunlight and heavy rain. Furthermore, the statistical analyses reveal that solar radiation is the principal factor guiding the temperature variation of surface water in summer. Owing to the decreasing solar radiation density with depth, atmospheric temperature dominates water temperature until the depth over 0.5 m. However, the intensity of solar radiation deposits in winter, resulting in atmospheric temperature dominating the temperature change of water column. It supports that the effect of thermal conduction and density flow on the changes of temperature profiles of the whole lake is the common factor in all seasons; radiant heat is the particular factor which primarily causes the temperature change of surface water in summer.