Abstract: According to the characteristics of nonlinearity and time delay of coagulant dosing in water treatment,it is difficult to realize a good result with traditional control method after analyzing the principle of coagulant dosing.A control scheme for coagulant dosing is proposed by combining generalized predictive control(GPC) algorithm with PID(proportion-integral -derivative) control algorithm.A new performance optimized function is got by predicting a number of controlled system's future outputs.And a reasonable coagulant dosage is computed by optimizing controlled object's parameters in real time.Finally,the scheme is coded with Visual Basic software.It gets a good output with local experiment on automatic control for coagulant dosing,which proves the feasibility of the control scheme.