Abstract: In this paper，the effect of methanol extract from three citrus peels on the growth of Microcystis aeruginosa PCC7806 was studied.The parameters，such as biomass，chlorophyll a concentration and chlorophyll a fluorescence，were used in the assay.The result showed that all the three extracts could inhibit the growth of M.aeruginosa effectively，as to the chlorophyll a concentration and the activity of PSⅡ.And the inhibitory ratio (IR) increased with the concentration of the extracts.The order of the inhibitory activity was as follows： Citrus unshiu>Citrus grandis>Citrus sinensis.In the mandarin group，the inhibitory effect was remarkable in the present of the concentration of the extract higher than 1.10 g/L (P<0.05)，which was kept in the whole period of experiment.And the IR of biomass is up to 86.4% after 9 days culture.When the extract concentration is higher than 3.31 g/L in navel orange and grapefruit groups，the inhibitory effect showed significant (P<0.05).However，the recovery happened after 5 days culture.It was deduced that there was some material in the citrus peels inhibiting the growth of M.aeruginosa by way of the inhibition of chlorophyll a synthesis and the activity of PSⅡ，which could both reduce the efficiency of the photosynthesis.And also the material could be degraded in natural conditions resulting in the inhibitory effects disappearing gradually with prolonged exposure.