In order to study the biodiversity of zooplankton and water ecological status in Baiyangdian lake, the paper investigated the zooplankton structure and aquatic ecologiy in June and October of 2012. The result showed that there are a total of 37 zooplankton species in Baiyangdian lake, most of which belong to cladocera which occupy 51.4% of the total. The total density of zooplankton at different sampling site varyies from 2480 ind./L to 7580 ind/L, with an average of 4160 ind/L. The biomass of zooplankton at different sampling site varies from 0.46 mg/L and 1.81 mg/L, with an average of 1.24 mg/L. Margalef index is in the range of 1.83-3.51.Zooplankton loss index showed that the conditions of zooplankton in Baiyangdian lake shows a decrease tendency and with different degree in each sampling sites compared with the historical condition. Baiyangdian lake is in a moderate pollution level and in a eutrophication state according to the comprehensive evaluation.