Abstract: During the process of electromagnetic detection technology applied in underground working face detection, it often encounters time delay phenomenon, and when the data inverts into a chart, it often encounters similar oval-shaped contour. The above phenomenons often affect the final delineation of anomalies. Aiming at the problems encountered in the work, this article detailedly descripted the causes of time delay phenomenon in underground penertration detection, as well as distribution limitation of electromagnetic detection technology applied in the underground, and the impact on abnormal area mapping.