With the rapid development of nanotechnology and widespread use of nanoproducts, concerns have arisen regarding the ecotoxicity of these materials. In this paper, the photosynthetic toxicity and oxidative damage induced by nano Fe₃O₄ on a model organism, Chlorella vulgaris (C. vulgaris) in aquatic environment, were studied. The results showed that Nano-Fe₃O₄ was toxic to C. vulgaris and affected its content of chlorophyll a, malonaldehyde and glutathione, CO₂ absorption, net photosynthetic rate, superoxide dismutase activity and inhibition of hydroxyl radical generation. At higher concentrations, compared with the control group, the toxicity of nano-Fe₃O₄ was significantly different. It suggested that nano-Fe₃O₄ is ecotoxic to C. vulgaris in aquatic environment.