Abstract: Microplastics, as a pollutant in the ocean, have become a global concern in recent years. The microplastic pollution in the East China Sea is not negligible. At the same time, Prorocentrum donghaiense is a common harmful alga that often blooms in the East China Sea. In order to elucidate the mechanism of the influence of polystyrene microplastics (PM) on the growth of this alga, this study conducted an experiment with different concentrations of PM exposure. The results showed that, first, high concentration of DOM (5 mg/L) promoted the growth of P. donghaiense. Second, 0.1 μm PM at concentration of 10 mg/L affected the growth of P. donghaiense in the early phase of growth, which was inhibited at the first and then promoted. Third, PM could inhibit the growth of P. donghaiense during the phases of stable and decline periods. The inhibition by microplastics became stronger with decrease of particle size and increase of concentration. The inhibition rate of the concentration is higher than the particle size. The effects of microplastic pollutant on the growth of P. donghaiense found in the present study can provide a new scientific knowledge for the marine environment protection.