High abundance of algae and eutrophication were observed in mangrove wetlands and these were estimated to be associated with root exudates of some specific mangrove plants to a certain extent.Root exudates form allelopathic effects from mangroves.The main secondary metabolites of Aegiceras corniculatum had been detected to be organic phenolic acids.Gallic acid had been isolated and identified from A.corniculatum.The half-maximal inhibitory concentration of gallic acid on alge Cyclotella caspia was tested as 15.46mg/L.The effects on algal cell morphology were mainly shown as elongated cells,with no apparent cell inclusions,such as oil droplets,chloroplast.At a dose of 2mg/L,gallic acid had a stimulative effect on the specific growth rate of algae on day 3.The contents of malondialdehyde,superoxide dismutase,soluble carbohydrates and chlorophyll a in algal cells showed an overall “low promotion and high suppression”.Our results could provide preliminary and valuable reference on the complex influences of mangroves on microecology and microbial communities in the rhizosphere system. High abundance of algae and eutrophication were observed in mangrove wetlands and these were estimated to be associated with root exudates of some specific mangrove plants to a certain extent. Root exudates form allelopathic effects from mangroves. The main secondary metabolites of Aegiceras corniculatum had been detected to be organic and organic acids were obtained from A.corniculatum. The half-maximal inhibitory concentration of gallic acid on alge Cyclotella caspia was tested as 15.46 mg / L. The effects on algal cell morphology were mainly shown as elongated cells , with no apparent cell inclusions, such as oil droplets, chloroplast. At a dose of 2 mg / L, gallic acid had a stimulative effect on the specific growth rate of algae on day 3. The contents of malondialdehyde, superoxide dismutase, soluble carbohydrates and chlorophyll a in algal cells showed an overall “low promotion and high suppression”. Our results could provide preliminary and valuable reference on the co mplex influences of mangroves on microecology and microbial communities in the rhizosphere system.