In order to investigate the toxic effect of light oil on marine phytoplankton, and provide theoretical basis for its prevention and control, we simulated the indoor toxicity stress experiment of light oil on two species of marine microalgae in different seasons (10, 16, 20 and 28 ℃), to research the effects of light oil on the growth of two species of marine microalgae. The results showed that light oil has certain inhibitory effect on the growth of two species of marine microalgae. The inhibition rate curve shows that the 96h inhibitory concentration of Prorocentrum donghaiense is 2.40 mg/L, and the 96 h inhibitory concentration of Heterosigma akashiwo is 4.10 mg/L. At the same time, because of the high volatile characteristics of light oil, with the concentration of light oil decreased in the course of the experiment, the growth rate of Prorocentrum donghaiense and Heterosigma akashiwo recovered. The effect of light oil on phytoplankton growth is also related with the temperature change. The higher the temperature, the more obvious effects, namely, the winter has minimum impact, while summer has the most obvious influence.