Abstract: In order to explore the toxicity and mechanism of heavy metals arsenic in aquatic ecosystem, Chlorella vulgaris, a typical phytoplankton indicator species, was used as the experimental object. Different concentrations of As⁵⁺ (0, 0.1, 0.5, 1, 1.5 and 2.0 mg/L) were used to study the effects of As⁵⁺ on the density, chlorophyll a content, cell membrane permeability, reactive oxygen species (ROS) content, malondialdehyde (MDA) content, total antioxidant capacity (T-AOC), total protein (TP) content, total superoxide dismutase (T-SOD) activity, ATP activity and ultrastructure of C. vulgaris. The results showed that all concentrations of As⁵⁺ could inhibit the growth of C. vulgaris within 96h, and the inhibition rate of 2 mg/L treatment group was the highest (52.9%). The IC₅₀ of 96h was 1.94 mg/L. The Chl.a content of algae cells was also inhibited in all treatment groups, among which, the treatment group of 2 mg/L was the lowest which was only 31.4% of the control group. When the concentration of As⁵⁺ was 1.5 and 2 mg/L, the content of TP and ATP decreased significantly. The content of MDA was 2 times and 2.6 times higher than that of the control group, and the cell membrane permeability was 1.7 times and 2 times higher than that of the control group. The content of ROS in 2 mg/L treated group increased significantly, which was 4.0 times of that in control group. The activities of T-AOC and T-SOD in algal cells stimulated by oxidative stress were 2.8 times and 1.6 times higher than those in control group. The results of transmission electron microscope showed that the group treated with 2 mg/L As⁵⁺ could destroy the internal structure of algae cells, resulting in cytoplasmic vacuolation, lamellar structure fracture of thylakoids and chloroplast structure disorder. The results showed that high concentration of As⁵⁺ was destructive to the structure and function of cell membrane and main organelles of C. vulgaris. At the same time, C. vulgaris produced oxidative stress in response to pollutant stress. The concentration of As⁵⁺ below 1.5 mg/L had no significant effect on physiological and biochemical indexes of C. vulgaris. This study reveals the physiological and biochemical reactions of the typical indicator species C. vulgaris under As⁵⁺ environmental stress, and provides basic data for the remediation of heavy metal pollution in the water environment and the formulation of relevant environmental standards, thereby realizing the sustainable development and utilization of the aquatic ecosystem.