Abstract: Allelopathy of culture water from ten kinds of macrophytes on Synahocystis sp.was investigated.The results showed that there was slightly stimulation effect for Ottelia acuminate culture water on Synahocystis sp.,while significant inhibition effects of other nine macrophytes on the algae were performed,and their inhibition capability was as following: M.aquaticumE.crassipesC.demersum P.stratiotes N.peltatumM.spicatum H.verticillataD.sanderianaV.spiralis,the inhibition ratio of M.aquaticum on Synahocystis sp.is up to 89.9%.By analyzing the absorption spectral curve and the characteristic peaks of Synahocystis sp.cell,it was found that M.aquaticum,Eichhornia crassipes and C.demersum could secrete some allelochemicals,which destroyed the chlorophyll a and phycobiliprotein absorption peak,and disturbing the course of light-harvesting of Synahocystis sp..It probably indicated that allelopathic compounds released by macrophytes restrain the photosynthetic system of algae,which was one of importance ways for macrophytes inhibiting or even killing algae.