Abstract: Based on laboratory cultivate experiments, this project studied the ecological toxic effects of norfloxacin, a marine environmental pollutant, on Dunaliella salina, Nitzschia closterium f.minutissima and Chlorella vulgaris.The results showed that, within the experimental concentration range, with the increase of the time, the biomass of the three microalgae increased, which conforms with the Logistic growth model.The toxicity effect of norfloxacin on the three kinds of microalgae was different, and this effect was the lowest in the Nitzschia closterium f.minutissima, the EC₂₀ and the EC₀₅ are 25.36 mg/L and 1.76 mg/L; the toxicity of Dunaliella salina is low, the EC₂₀ and the EC₀₅ are 10.54 mg/L and 1.25 mg/L; the toxicity effect of Chlorella vulgaris is the highest, the EC₂₀ and the EC₀₅ are 5.33 mg/L and 0.01 mg/L.With the increase of concentration, the increase in the inhibitory rate of norfloxacin to three kinds of marine microalgae decreased.In addition, at 1 mg/L, the B_f of Nitzschia closterium f.minutissima was slightly higher than that of the control group, which may be related to the toxic excitatory effect.With the increase of norfloxacin concentration, the exponential growth period of Chlorella vulgaris was significantly shortened.Based on the species sensitivity distribution, the predicted no effect concentration (PNEC) of the norfloxacin contaminant to marine ecosystem was 0.096 mg/L.