Abstract: Effects of the pre-dam system built in the northwest of the Gehu Lake, where the Xiaxi River and the Biandan River merged and flowed into the lake, were investigated. Results show that the effects of the system were significant on removing of N, P and algae from inflow rivers. During the stable period, the pre-dam system raised the quality of the inflow water from Gategory Ⅴ_minus to Gategory Ⅲ-Ⅳ, and reduced the concentrations of TN ( total nitrogen ) and TP ( total phosphorus ) from 1.21-3.09 and 0.05-0.24 mg•L^-1 to 0.62-1.26 and 0.03-0.09 mg•L^-1, respectively, with average TN and TP removal rates being around 60%. For algae, the concentration of Chl-a ( chlorophyll-a ) was lowered from 15.10-126.07 to 14.32-42.95 mg•m^-3, with average removal rate being (67.55±3.06)%, and maximum removal rate reaching 82.82%. Besides, within the system, the effects of the storage buffer zone removing TN, TP and Chl-a were more obvious than those of the other zones ( P＜0.05 ). In the same zone inconsistent in water quality index, only the ecological stable sub-zone had an obvious effect on controlling Chl-a, and differed significantly from the other sub-zones in TN and TP removing effect( P＜0.05 ) .