This paper introduced the control system and composition of the high and low pressure bypass in No5 and No6 300 MW once-through boiler power generation units in Huaneng power plant as examples. There were some problems in this system, such as unsatisfactory function and accuracy of the control system, and aging single loop controller. It was necessary to take measures for these problems, which included optimizing control logic, regulating control parameters, replacing on site single loop controller, perfecting interlock protection function and so on.