The modified wheat straw (MWS) was prepared and used to investigate the removal and the mechanism of Heterosigma akashiwo. The results showed that under the same loading of 0.10 g/L, the algal removal efficiency could reach 80% by MWS in 120 minutes, while 10% by the unmodified wheat straw. To investigate the mechanism, we measured the optical density of the supernatant at 260 nm. The result showed that the nucleotides were released, which meant disruption of the cytoplasmic membrane. More than 15% of the nucleotides were released from the cytoplasm under the effect of 0.15 g/L of MWS, indicating the irreversible damage on the cellular membrane, which resulted in the disintegration of the harmful algal cells. It is suggested that the low concentration of MWS might adsorb onto the algal cell surface and bind to the cytoplasmic membrane, which resulted in the flocculation. While the high concentration might cause the penetration into the cellular membrane, which resulted in the leakage of cytoplasm material and hence the death of cell.