Abstract：The EH-4 electromagnetic imaging system has the advantages of great exploration depth, direct perceivability of the result, portability and high efficiency. This paper describes the basic principle of the EH-4 electromagnetic imaging system as well as the technology for data acquisition and data processing. The application of this system to the detection of the extension of the structure at depth in western Yunnan is also reported. Practice shows that the application of the EH-4 electromagnetic imaging system to the detection of the extension of the structure at depth is effective.