Abstract: Dinoflagellate Prorocentrum donghaiense is the most dominate harmful algal bloom (HAB) forming species in the coastal zones of East China Sea, according to the China Marine Environmental Quality Bulletin over the years, which has caused a huge damage to local aquatic systems and brought disaster to the aquaculture industry. In addition, blooms formed by P. donghaiense occur frequently in May every year, coincide with the sexual reproduction period of Sargassum fusiformis. Small-to medium-sized HABs have been occasionally observed to invade into the aquaculture area S. fusiformis. To evaluate the effects of HABs formed by dinoflagellate P. donghaiense on S. fusiformis and explore its mechanism, zygotes of S. fusiformis were cultivated with living cell suspension (LC), ruptured cell suspension (RC) and cell-free supernatant (FC) of 1.00×10⁵ cell/mL P. donghaiense to measure growth rate and photosynthetic activities according to changes in sizes and chlorophyll (Chl.) a fluorescence transient (i.e., OJIP) of the zygotes. The results showed that LC, RC and FC of P. donghaiense at 1.00×10⁵ cell/mL all significantly inhibited the development, growth, Chl. a contents, apparent photosynthetic efficiency (α), and the maximal electron transport rate (rETR_max) of the zygotes with greater inhibition by FC and LC. For other important photosynthetic parameters of the zygotes obtained through JIP-test analysis, FC and LC also greater reduced the maximum quantum yield (φp_o), the performance index (PI), and the active reaction center density than RC. It indicated that the decrease in photosynthetic activities of the zygotes was caused by the allelocamicals released by P. donghaiense, and some reaction centers (RCs) in the zygotes exposed to the allelocamicals were inactivated, which led to higher dissipation of excitation energy. RC of P. donghaiense contained more allelochemicals than LC and FC because they were released into the media during cells lysis. Therefore, some other compounds that can stimulate the growth and photosynthetic activities of the zygotes were synchronously released, which partly offset the inhibitory effects of the allelopathic substances. These results indicate that the inhibitory effects of P. donghaiense on S. fusiformis zygotes were mainly caused by allelochemicals, and the blooms formed by P. donghaiense can restrain the development and photosynthetic activities of S. fusiformis zygotes, reduce the seedlings stock, and eventually hinder the development of S. fusiformis production industry.