The invention relates to an extracting method of transient electromagnetic pure abnormal signal. The method comprises the following steps: determining a background field induced voltage data; determining a measuring point induced voltage data; deconvoluting each measuring point induced voltage data on a measuring line with the background field induced voltage data, removing a secondary field background field data, obtaining a secondary field pure abnormal field data without a secondary field background field; determining the character of an underground geological target based on the secondary field pure abnormal field data. The extracting method of transient electromagnetic pure abnormal signal greatly increases the precision of explaining the character of the underground target by removing the secondary field background field from the measuring point induced voltage data which is generated by the underground medium, i.e., extracting the abnormal field portion of the secondary field, and then determining the information based on the abnormal field of the secondary field.