A total of 156 species and 11 varieties of phytoplankton species were identified from a shallow reservoir, Lake Jinyang,in Shanxi Province, North China, representing 5 divisions, 25 families and 67 genera. Among them, Chlorophyta was the domi-nant division, a total of 57 species and 4 varieties of 29 genera. The second domination was Bacillariophyta, 34 species and 6 vad-eties of 17 genera. The next dominations were followed by Cyanophyta and Euglenophyta, with 32 species and one variety of 15 gen-era and 31 species of 5 genera respectively. Xanthophyta was the least one, only 2 species of one genus. The number of species washighest in autumn and lowest in winter. The range of cell density was 2.84×10⁶-2.30×10⁹cells/L, and the average density was4.86×10⁸frcells/L. The density of phytoplankton has reached the eutrophication level in every season. Both the diversity index andevenness index were low, but the dominant index was high. It was showed that this lake has become eutrophic and seriously polluted.