Abstract: In the process of aquaculture, a large amount of nitrogen and phosphorus is input into the aquaculture water body by feeding and fish excretion, which causes serious pollution to water body. As an important primary producer in aquatic ecosystem, microalgae play an important role in water environment management. Using microalgae to treat wastewater can not only purify wastewater but also obtain high value-added products. Microalgae can efficiently remove nitrogen, phosphorus and other pollutants in wastewater. At the same time, the construction and operation cost of the treatment process is low, therefore, it has been widely concerned. At present, studies on the treatment of aquaculture wastewater by microalgae have been reported. The feasibility of the treatment of aquaculture wastewater by microalgae has been proved. However, the effect of initial inoculation concentration of microalgae on the treatment efficiency of aquaculture wastewater has not been reported. In the present study, the response of growth and pollutants removal of Chlorococcum sphacosum GD to initial inoculation concentration (25—400 mg/L) was investigated in aquaculture wastewater based on physical and chemical analysis. The results showed that C. sphacosum GD with 100 mg/L of initial inoculation concentration had the best growth capability. At the end of the cultivation, the specific growth rate of the microalga inoculated with 100 mg/L of biomass was 0.1/d and the doubling time was 6.84d. With the further increase of initial inoculation concentration to 400 mg/L, the specific growth rate decreased to 0.015/d and the doubling time increased to 44.8d. Regression analysis showed that the initial inoculation concentration of the microalga was significantly negatively correlated with specific growth rate, and positively correlated with doubling time. Wastewater treatment efficiency also responded to the initial inoculation concentration. When the aquaculture wastewater was inoculated with 100 mg/L of biomass, C. sphacosum GD could remove 96.92% COD, 98.08% ammonium, 98.67% nitrite, 91.42% nitrate and 98.36% total phosphorus in 5-day cultivation period. The wastewater treatment efficiency of the microalga with 100 mg/L of inoculation concentration was obviously better than that of the microalga with other inoculation concentrations. The low initial inoculation concentration (especially 100 mg/L) was beneficial to the growth and pollutant removal of C. sphacosum GD. In addition, the chlorophyll content of the microalga increased significantly compared to initial cultivation, which was independent of initial inoculation concentration. Chlorophyll fluorescence values of F_v/F_m and F_v/F_o also maintained at a high level. Microalgal cells had good physiological activity in the process of wastewater treatment. Based on above results, in order to effectively remove pollutants by the microalga, it is suggested that the microalga should be inoculated at a low initial concentration of 100 mg/L biomass for the treatment of aquaculture wastewater. The microalga can be used for aquaculture wastewater treatment with the appropriate initial inoculation concentration. By controlling the initial inoculation concentration of microalgae, it is expected to improve the removal efficiency of pollutants, shorten the cultivation period and increase the volume loading. It also provides theoretical support for the application of microalgae in the large-scale treatment of aquaculture wastewater.