Abstract: Phaeodactylum tricornutum is one of the microalgae that cause red tide. This study explored the effects of different doses (100, 500 and 1000 μL) of Bacillus amylolyticus, B. amylolyticus metabolites and the mixture of B. amylolyticus and the metabolites on the growth of P. tricornutum. The results showed that metabolites and the mixture of B. amylolyticus and metabolites inhibited P. tricornutum. The numbers of algae cells in all 3 experimental groups were significantly reduced than those in the control group at day 4 (P<0.05). The metabolite and the mixed solution significantly reduced the number of algae cells from day 1 to 4 (P<0.05). Low and medium dosage of Bacillus amylolyticus significantly reduced the number of algae cells at day 3 and day 4, respectively. The number of algal cells in medium and high dosages of Bacillus amylolyticus further decreased with the addition of B. amylolyticus metabolites with a concentration dependent pattern. B. amylolyticus and its metabolites co-regulated P. tricornutum. under certain threshold with the highest inhibition rates of 37.36%, 54.87% and 65.05% respectively. In conclusion, the allelopathy of B. amylolyticus and its metabolites on the growth of P. tricornutum provided scientific basis for the effective control of red tide algae. In summary, Bacillus amyloliquefaciens has no obvious inhibitory effect on Phaeodactylum tricornutum, but its high concentration of metabolites and the mixture of the two can effectively inhibit the growth of Phaeodactylum tricornutum, and there is a threshold. Below this threshold, there is a concentration effect. The higher the concentration, the more obvious the inhibitory effect. The inhibitory effect of Bacillus amyloliquefaciens and its metabolites on Phaeodactylum tricornutum can be superimposed, and the inhibitory effect is the strongest when the two exist at the same time.