Wastewater treatment effect, together with species composition, cell densities and diversity of phytoplankton, was inves-tigated in biological stabilization ponds of Daqing River. The results showed that pH, DO and chlorophyll-a concentration increasedgradually, and TN,TP, NH₄⁺-N, BOD₅ and COD decreased gradually when wastewater flew from the first pond to the last pond.The removal rates of TN, TP, NH₄⁺-N,BOD₅ and COD in biological stabilization ponds were 29. 29%, 48.68%, 33.68%,68.14% and 71.25%, respectively. Chlorophyll-a concentration was correlated significantly positively with pH(r=0. 955,P<0.05)and DO(r=0.992,P<0.01),but significantly negatively with'TN (r=-0.936, P<0.05),TP(r=-0.925,P<0.05)and NH₄⁺-N(r=-0.927,P<0.05).A total of 53 algal taxa including six dominant species were identified in biologicalstabilization ponds. Algal species and diversity increased gradually and total cell densities decreased gradually in the system ofponds. The cell density of Microcystis viridis and Microcystis wesenbergii decreased gradually, and that of Cyclotella meneghiniana,Cryptomoaas erosa, Dictyosphaeriam palchellura and Sphaerocystis schroeteri increased gradually.