Abstract: According to the influence analysis on effluent present condition and water PH value stability of Xiaolangdi reservoir to hydro-junction safety operation, the layout and design necessity of water quality online monitoring system of Xiaolangdi hydro-junction area is demonstrated. The water quality online monitoring stations are advocated to set in front of and behind Xiaolangdi dam and behind Xixiayuan dam. The monitoring parameter and monitoring equipment function should be analyzed.