Previous study indicated that wheat bran leachate(WBL) exhibited strong inhibitory effect against Microcystis aeruginosa.In order to elucidate the responses of M.aeruginosa photosystem to the stress of WBL,pigments and chlorophyll fluorescence transient of M.aeruginosa CHAB-109 under the stress of WBL were analyzed.The results indicated that intracellular pigment contents decreased when WBL was of 1.6 and 3.2 g/L.Results of chlorophyll fluorescence transient analysis indicated that the absorbed energy(ABS/RC),the trapped energy for reduce of Q_A (TR₀/RC),the trapped energy for electron transport(ET₀/RC) were all inhibited by WBL.Energy flux ratio analysis of PSⅡ indicated that no damage induced by WBL at the electrons providing side of reaction centre of PSⅡ,but it can damage the acceptor side of reaction centre.Combining the results of energy fluxes and flux ratios indicated that the electron transport chain from reaction centre to Q_A and far beyond Q_A are the damage sits of WBL on Microcystis aeruginosa.