Abstract: In recent years, frequent cyanobacterial blooms induced by freshwater eutrophication have posed severe threats to aquatic ecosystem security. Traditional physical and chemical algae control technologies suffer from drawbacks such as secondary pollution and poor selectivity, necessitating the urgent development of novel environmentally friendly algicides. 3-Hydroxydecanoic acid （3-HDA）, a short-chain fatty acid derivative, is environmentally friendly and exhibits promising algicidal potential. To address the harm of cyanobacterial blooms, make up for the application shortcomings of conventional algae control technologies, and develop high-efficiency and environment-friendly algae control substances, this study focused on Synechococcus sp. PCC7942, a typical cyanobacterium, to investigate the inhibitory effects and mechanisms of 3-HDA. Exponential-phase Synechococcus sp. PCC7942 （cultured in BG-11 medium at 26 ± 1℃ under a 12 h:12 h light-dark cycle） was exposed to gradient concentrations of 3-HDA （0, 0.5, 1, 5, 10, and 25 mg/L）. Algal cell growth （OD₇₃₀, inhibition rate）, photosynthetic pigments （chlorophyll a）, metabolites （soluble protein, soluble sugar）, damage markers （malondialdehyde, MDA）, and activities of antioxidant enzymes （superoxide dismutase, SOD; peroxidase, POD; catalase, CAT; polyphenol oxidase, PPO） were dynamically monitored over 24-168 h to evaluate 3-HDA is potential as an eco-friendly algicide. Results demonstrated that 3-HDA exerted concentration- and time-dependent inhibitory effects on Synechococcus sp. PCC7942, with a 72 h EC₅₀ of 5 mg/L. Chlorophyll a content decreased significantly, soluble protein synthesis was suppressed, and soluble sugar accumulation was blocked. Additionally, MDA levels increased, while SOD, POD, CAT, and PPO activities first rose and then declined, all remaining significantly higher than the control group. In conclusion, 3-HDA inhibits algae through a multi-target mechanism involving disruption of photosynthetic system function, inhibition of key metabolic pathways, and induction of oxidative damage. Its low-dose efficacy, high efficiency, and environmental friendliness position it as a promising candidate for the development of novel ecological algicides.