Qiandao Lake located in the upper reaches of Qiantang River is a large deepwater reservoir in north subtropical zone of China. In the lake, the water temperature reportedly has the vertical stratification phenomenon, and it presents thermocline in the summer. Cladocera is an important group in the reservoir ecosystem, it is generally recognized that Cladocera possesses the behaviors of vertical distribution and diel migration. Then the area next to the dam (with the total water depth exceed 60m) was selected as the sampling site, the environment factors and cladocerans species in vertical layers were analyzed in four seasons (from September 2004 to August 2005). The specie Daphnia hyaline was chosen as the main research object in order to clarify the influence of seasonal and vertical distribution of water column temperature on the vertical distribution pattern and diel migration of cladocerans in Qiandao Lake. The results show that D. hyaline is the dominate specie all year around, and essentially has an aggregated distribution pattern (MI>1) in water column, and with normal migration all year around except autumn. The thermocline in Qiandao Lake strengthens the aggregated distribution of D. hyaline in water column, promotes the upward-migration in the night. The fact makes the specie D. hyaline the important link between top and bottom waters for material interchange and energy flow.