The invention relates to an 
algaecide. The 
algaecide which can be used for effectively removing 
algae and precipitating alga bodies contains 
chitosan, 
fly ash autoclaved 
brick powder and allelopathic chemicals of 
gallic acid, nonoic acid or N-phenyl-
naphthylamine, and is obtained by dissolving a certain amount of 
chitosan by using 
hydrochloric acid, then adding the 
fly ash autoclaved 
brick powder of 40-200 meshes, and then adding the 
dimethyl sulfoxide assisted dissolved allelopathic chemicals of 
gallic acid, nonoic acid, N-phenyl-1-
naphthylamine or N-phenyl-2-
naphthylamine. The 
chitosan and the 
fly ash autoclaved 
brick powder used in the 
algaecide provided by the invention are nontoxic, the allelopathic chemicals of 
gallic acid, nonoic acid and the like belong to 
plant secondary metabolism products, can be degraded easily in a 
water body and cannot cause secondary 
pollution, and although the diluted HCl solution is used for dissolving chitosan, great changes of the pH value of the 
water body are not caused in a dosage range. The algaecide provided by the invention can be used for removing the 
algae effectively and ensuring that the alga bodies are sunk to the bottom of water, has a small influence on the environment and does not cause secondary 
pollution, the 
water body is clear, and the beautiful outlook of the water body is not influenced, so that the algaecide is suitable for removing the 
algae from a small landscape water body taking blue algae as a dominant species.