Abstract: The wide application of prometryn and other triazine herbicides in agricultural production and mariculture makes the detection rates of triazine herbicides in coastal seawater relatively high. Due to its stable structure triazine ring, prometryn has a long half-life, which makes it easily accumulated in organisms and cause toxicity. In this study, Isochrysis galbana was used to evaluate the toxicity of prometryn at different environmental concentrations (0.5, 1.0 and 2.5 µg/L). The results showed that the growth of Isochrysis galbana was inhibited by prometryn, and the algae density in the1.0 and 2.5 µg/L prometryn groups was 78.3% and 35.3% of that in the control group. At the same time, the content of chlorophyll a was decreased by 19.2%, 23.2% and 50.1%, and carotenoids was decreased by 20.4%, 19.7% and 51.7% in the 0.5, 1.0 and 2.5 µg/L prometryn groups. Meanwhile, the content of chlorophyll b was decreased by 38.3% after exposure to 2.5 µg/L prometryn. Moreover, exposure to 2.5 µg/L prometryn decreased the relative electron transport rate (rETR) by 57.2%, actual photochemical efficiency Y(Ⅱ) by 57.0%, maximum quantum yield of light (Fv/Fm) by 30.0%, and non-photochemical quenching (NPQ) also decreased significantly. Exposure to 2.5 µg/L prometryn, the accumulation of carbohydrate, triglyceride and soluble protein were decreased by 49.2%, 50.2% and 26.8%, and the proportion of C and N elements also decreased by more than 30%. In addition, the morphology of algal cells in the prometryn exposure groups showed abnormal structures including depression, folds and holes. The results of this study can provide a scientific reference for assessing the ecological risk of triazine herbicides pollution.