Abstract: Briefed in this article are the characteristi cs and application of the industrial PROFIBUS-DP technology, and the equipment composition and manufacturing process flow of the straight welded pipe expanding mill.Also detailed is how the Siemens S7-400-based electrical control system for the said pipe expander is used to realize the instant communication between the PLC and 6SE70 inverter, and the network self-diagnosis via the PROFIBUS-DP technology.The operation practice shows that since using of the PROFIBUS-DP system for the production line, the straight welded pipe mill has been running smoothly, and so has been the electrical control system itself which features quick-response, high control accuracy, consistent network communication, and very few occurrence of "network crush".