Abstract: Heterosigma akashiwo is a typical red tide alga. To elucidate mechanisms of harmful algal blooms (HABs) by Ulva fasciata, H. akashiwo was exposed to gradient concentrations of 0.6, 1.2, 2.4 and 4.8 g/L dried U. fasciata powder, and then measured the photosynthetic efficiency of H. akashiwo by plant efficiency analysis (PEA) and its ultrastructure via transmission electron microscope (TEM). The results showed that dried U. fasciata powder significantly reduced the content of photosynthetic pigments in H. akashiwo and inhibited its growth. At 72 hours, the inhibition rates of treatments from low to high concentration were 19.6%, 46.5%, 71.4%, and 79.4%, respectively. Dried U. fasciata powder significantly reduced the photosynthetic efficiency of H. akashiwo and reduced its OJIP curve at 72 hours with a dose-dependent pattern. Compared to control group, 4.8 g/L treatment group increased the energy absorbed into per photosynthetic reaction center (ABS/RC) in H. akashiwo and the energy used for deoxygenate Q_A (TR_o/RC) 15 times and 1.5 times, respectively. Simultaneously, 4.8 g/L treatment group reduced the energy consumed for electron transmission in per reaction center (ET_o/RC), the maximum photochemical efficiency (ψP_o), the quantum yield of electron transfer (ψE_o), and the ratio of electrons exceeding Q_A (Ψ_o) by 81.3%, 81.2%, 91.7% and 81.7%, respectively. Treated with 4.8 g/L dried U. fasciata powder for 72 hours, the thylakoid lamellas of H. akashiwo were broken and arranged disorderly, thylakoid membranes were fused, chloroplast outer membranes were ruptured, mitochondria and cell nuclei were ruptured, and cell membrane structure was destroyed. This result indicated that dried U. fasciata powder inhibited the photosynthetic reaction center and electron donor function, and the transmission of electrons was blocked. Observation through TEM showed that after 72h treatment with 4.8 g/L dried U. fasciata powder, the thylakoid lamellas of H. akashiwo were broken, arranged disorderly, thylakoid membranes were fused, chloroplast outer membranes were ruptured, mitochondria and cell nuclei were ruptured, cell membrane structure was also destroyed. In closing, dried U. fasciata powder can exhibit obvious destructive effect on the photosynthesis of bloom-forming H. akashiwo, and inhibited its normal growth, thus, U. fasciata could serve as a ‘tool alga’ to control and mitigate H. akashiwo blooms, while more intensive and practical research on its biological controlling of HABs should be furtherly made.