Abstract: In this paper, application of PLC control system in semi-dry flue gas desulfurization project of the boiler plant in the second mine of Cuncaota, ShenDong was introduced. The process principle of semi-dry flue gas desulfurization was overviewed, the selection and components of the PLC control system, as well as the reliability design of power supply and ground connection were illustrated. The control system was divided into three sections, which were DAS (Data Acquisition System), MCS (Modulating Control System) and SCS (Sequence Control System) according to the functions, at the same time, the control object and principle of each section were discussed in detail. PID control principle was also applied in MCS to realize close-loop control.