Abstract: In this study, by taking Isochrysis galbana as the research object, the growth and the changes of fatty acid composition of Isochrysis galbana were analyzed under the treatments of different concentrations of enrofloxacin. The growth changes of Isochrysis galbana were measured by using the cell counting method and spectrophotometric method, and the composition and content of fatty acid in Isochrysis galbana were measured by using the gas chromatographic method. The results showed that enrofloxacin exhibited a low-promoting and high-inhibiting effect on the growth of Isochrysis galbana, namely, showing a promoting effect when the concentration of enrofloxacin was ≤ 0.625 mg·L^-1, and showing an inhibiting effect when the concentration of enrofloxacin was ≥ 5 mg·L^-1, but there was no concentration-effect relationship. Under the stress of enrofloxacin, the composition and content of fatty acid in Isochrysis galbana changed. With the increase of incubation time, the total content of fatty acid in Isochrysis galbana decreased, and when the incubation time was 24 h and 96 h, the content of unsaturated fatty acid in each concentration group was lower than that in the control group (the concentration of enrofloxacin was 0 mg·L^-1), and the content of DHA was not significantly different from that in the control group. When the concentration of enrofloxacin was ≥ 5 mg·L^-1 and the incubation time was 240 h, the contents of unsaturated fatty acid and DHA in Isochrysis galbana were significantly higher than those in the control group. Therefore, the proportion of unsaturated fatty acids in Isochrysis galbana was increased under the stress of enrofloxacin.