The electromagnetic tomography technique is a new widely-used geophysical method. It is highly accurate, rapid and economical. This paper describes in brief the primary theory and working procedure of electromagnetic tomography. Combined with several engineering examples, this paper illustrates the structural forms of underground media relatively intuitionally and clearly by the form of 2D image. This result provides an important basis for the engineering geological survey as well as the evaluation of the safety and stability of site bases and dams.