We classified the zooplankton functional groups in a freshwater ecosystem, the Sanhuanpao Wetland Reserve, to determine the seasonal variation related to the environmental factors. We investigated the zooplankton in spring, summer, and autumn, and identified seven functional groups during the study period. Group RF (Rotifers filter feeders) was dominant in spring and was important to control the phytoplankton. Groups RC (Rotifers carnivore), PF (Protozoas filter feeders), RF, and LCC(Large copepods carnivore) were dominant in summer, whereas groups RC and RF were dominant in autumn. Group RC obviously inhibited the growth of PF and RF in summer and autumn. The results from Pearson and RDA analyses revealed that interspecificcompetition, Cl^-(Chloride ion), TN(Total nitrogen), IC (Total Inorganic Carbon), and BOD₅ (Biological oxygen demand) were the most important factors affecting zooplankton functional groups in the Sanhuanpao Wetland Reserve. Cl^-, TN, and IC showed a positive relationship with all the zooplankton functional groups except RF, whereas BOD₅ showed a negative relationship with all the zooplankton functional groups except RF. The functional groups SCF, MCF, MCC, and LCC had a positive relationship with each other, whereas group RC showed no relationship with groups PF and RF. Interspecific competition, bottom-up effects of phytoplankton and nutrients were the most important factors affecting the seasonal variation and biomass of zooplankton functional groups in cold region of marsh wetland.