Abstract: IN this paper, it introduced a control method to coagulant dosing against nonlinear and time-delay in the process of coagulant dosing. It applied generalized predictive control(GPC) algorithm and PID algorithm, online optimized relative parameters and got a reasonable coagulant dosage. Finally, through online experiment on automatic control to coagulant dosing, it got a good output and proved the practicability of the control method.