The physicochemical environment and phytoplankton species composition and density in 9 Macrobrachium nipponense mid-cultured waters in Siyang City, Jiangsu Province were analyzed through sampling on July 12, 2018, and the effects of different culture modes on culture environment were compared. Results showed dissolve oxygen (DO, > 8.44 mg/L) and pH (7.91 ~ 9.26) were relatively high, while total nitrogen (TN, 1.030 ~ 1.571 mg/L), total phosphorus (TP, 0.174 ~ 0.421 mg/L) and chemical oxygen demand (CODMn, 4.39 ~ 8.16 mg/L) were low in the Macrobrachium nipponense cultured waters. A total of 64 genus/species of phytoplankton were observed in the cultured waters, and most of them were species belonged to Cyanophyta and Chlorophyta. The diversity index of phytoplankton was high and the community structure was stable. The density of phytoplankton was 0.06 ×108 ~ 3.06 ×108 cells/L. Results of RDA showed that water temperature, NO2--N and CODMn were the main environmental factors influencing the density of dominant phytoplankton in the Macrobrachium nipponense cultured waters. Different species of mixed culture have certain influence on the physicochemical indexes and phytoplankton in the Macrobrachium nipponense cultured waters, but the influence of management mode on environmental factors is more significant. Since the concentrations of pH and Ca2+ in the investigated Macrobrachium nipponense cultured waters were lower than the growth-optimizing values, and high concentrations of TP and CODMn promoted the outbreaking of cyanobacterial bloom, it is recommended to apply appropriate quicklime to increase the concentrations of pH and Ca2+, and take necessary steps to control the increasing of TP and CODMn in the cultured waters.