Quinolone antibiotics, especially enrofloxacin and ciprofloxacin, are currently found to be at high levels commonly in natural waters. Studying the effects of these two quinolones on the growth of common microalgae in water is valuable for understanding the ecological effects of quinolones in the aquatic environment. Therefore, the effects of different concentrations of enrofloxacin and ciprofloxacin hydrochloride on the population growth of four species of planktonic microalgae (Chlorella vulgaris, Tetradesmus obliquus, Scenedesmus quadricauda, and Microcystis aeruginosa) were studied. The results showed that there was a significant effect-dose regression relationship between the concentration of nofantibiotic and the growth inhibitory rate of the algae population. The 96h half maximal inhibitory concentration (96h-IC50) of enrofloxacin on T. obliquus, S. quadricauda, M.aeruginosaI and C. vulgaris were 195.6, 88.8, 56.1, and 22.6 mg/L respectively, and the order of sensitivity of the four microalgae to enrofloxacin was T. obliquus＜S. quadricauda＜M. aeruginosa＜C. vulgaris. Another 96h-IC50 of ciprofloxacin hydrochloride to S. quadricauda, T. obliquus, M. aeruginosa, and C. vulgaris were 588.6, 546.3, 49.8, and 44.7 mg/L respectively. The sensitivity of the four microalgae to ciprofloxacin hydrochloride was ranked as follows: S. quadricauda＜T. obliquus＜M. aeruginosa＜C. vulgaris. Among the four species of planktonic microalgae, two species of Scenedesmus were less sensitive to the quinolone antibiotics. Therefore, these two species of Scenedesmus were more resistant to the two quinolone antibiotics, while C. vulgaris was the most sensitive to them. According to the data of IC50, the toxicity of enrofloxacin to three green microalgae was higher than that of ciprofloxacin, while the toxicity of enrofloxacin to M. aeruginosa was lower than that of ciprofloxacin. The population growth of the four planktonic microalgae was inhibited in the first 4 days after being treated by these two quinolone antibiotics, but the microalgae could gradually detoxify and restore their population growth after 96 h. Moreover, the toxicity of enrofloxacin to the four algae increased with time. The toxicity of ciprofloxacin hydrochloride to T. obliquus and S. quadricauda increased, and the toxicity to M. aeruginosa and C. vulgaris decreased. The recovery degree of ciprofloxacin hydrochloride was at least 39% higher than that of enrofloxacin. Low concentrations of enrofloxacin (<36mg/L) provoked a higher density of M. aeruginosa than that of the control group after 10 days of exposure. In this case, the algal density of M. aeruginosa could be more than 107 cells/L when the concentration of enrofloxacin was below 36mg/L.