Abstract: Allelopathic algal inhibition is one of important mechanisms for submerged macrophytes to gain a competitive advantage and stabilize clear-water states. Allelochemicals are secondary metabolites produced and released into water environment by submerged macrophytes, which are pivotal for submerged macrophytes to exert allelopathic algal inhibition. Hence, it is very important to clarify their modes of action, process and mechanisms of allelochemicals. Varieties and contents of allelochemicals were released by submerged macrophytes, and their algal inhibitory effects were reviewed, indicating that the modes of action for allelochemicals at releasing levels were different from a single function in traditional acute toxicity assays. To figure out how allelochemicals at releasing levels to exert effective algal inhibition, combined and continual action as two possible modes of action were discussed. It was proposed that multiple allelochemicals of submerged macrophytes might be released continuously at low-dosage levels to exert a persistent and synergetic inhibition on target algae. In the future research, plant-algae coculture system, in situ experiments, analytical chemistry, plant chemistry, cell biology, and molecular biology are extremely essential for understanding allelopathical mechanisms of submerged macrophytes on phytoplankton.