This article introduce the principle and characteristic of Dual-alkali that extensively used to desulfurize in Small and Medium boiler, adopt Fuzzy-PID control unit, the pH value of clean water that discharged from the Desulfurization cleaner have been effective controlled, therefore, it is arrive at the aim of indirect control of desulfurization efficiency. The actual operation proved that the Fuzzy-PID have good performance of static control and dynamic control, it is have preferably control effect for time delay and great inertia nonlinear process control.