Piston reciprocating compressors are the key to the petroleum and chemical industry. They normally control air flow with bypass return flow but have disadvantage of high energy consumption. Based on their energy saving reconstruction with stepless regulation system, this paper explains how air flow is regulated and how pressingoff inlet valve in partial stroke is used to control the flow. The system uses S7200 SMART PLC as slave machine and SMART 1000 IE touch screen as master one. It has signals from temperature and pressure sensors collected via PROFIBUS bus. Together with control program developed by sequential PID algorithm, air flow could be continuously controlled by an inlet valve with open range from 0 to 100%. Through the industrial application, this system shows its precise control over air flow and its considerable reduction in energy consumption.