Through lab acoustic measurement, this paper explores the features of backscattering ultrasound waves in Prorocentrum donghaiense (P. donghaiense) at different concentrations, and establishes the correlation curves between the integrated backscatter (IBS), an indicator of backscattering intensity, and P. donghaiense concentration. The experimental results show a highly linear correlation between the IBS with low concentration, and a reduced linear correlation between the IBS with high concentration; the reduction is attributable to the acoustic attenuation effect of dense algae particles in the high concentration solution. Then, the acoustic measurement performance was improved through particle size correction, leading to a good overall characterization effect. Finally, the proposed method was proved valid through data analysis. The research findings shed new light on acoustic monitoring of phytoplankton concentration, and provide a good reference to the forecast, prevention and control of marine red tides.