In Lake Taihu, Daphnia population dominated in the cladoceran community in winter and spring. However, the population disappeared in summer and autumn when it was replaced by the small-sized cladocerans. Theoretically, temperature and phosphorus concentration were two Limited factors for the Daphnia population. It is hypothesized that high temperature and phosphorus concentration were the reasons for the population decline of Daphnia in summer and autumn in Lake Taihu. The two experiments simulated summer and autumn separately were conducted. The results indicated that D. similis had relatively high increasing population rates under the simulating conditions, which proved that the high temperature and phosphorus concentration in summer and autumn in Lake Taihu were not the limit factors for the population decline of Daphnia. It was also presumed that the planktivorous fish predation and Microcystis bloom may be the reasons of the population decline of Daphnia in Lake Taihu.