Enrofloxacin is a widely used antibiotics in animal production. Animal excreted residual enrofloxacin which can adversely affecting the environment. The effects of 5 concentrations of enrofloxacin (0, 0.2, 0.5, 1, 2, 5mg•L^-1) on the aquatic microorganisms and their degradation rates were determined in a constructed wetland to assess the ecological risk of enrofloxacin on aquatic ecosystem. The results showed that enrofloxacin was rapidly degraded in the wetland environment in the initial stage, with more than 50% degraded within the first 5hours. And thereafter the degradation rate of enrofloxacin slowed down. The results indicated that enrofloxacin concentrations of 0.2～5mg•L^-1 had no significant effects on the population of aerobic bacteria, fungus, actinomycetes, nitrite bacteria and nitrate-reducing bacteria under aquatic environment. The ecological effects of enrofloxacin on aquatic ecosystem were also discussed.