Abstract: As the air compressors and diesel fire pumps of a power plant are inherently equipped with PLC control system, and its secondary circuit design is compound, faults are easily caused and difficult for troubleshooting, which brings about potential dangers to the safe operation of units. With these considerations,technical transformation is carried out by retaining a simple and reliable secondary circuit as the sole onsite for the above equipment, canceling the design of local "black box" PLC, and put all control logics together contained in the DCS control system, so as to improve the independent controllability and reliability of the control system.