The early quadratic field of the transient electromagnetic method is the response of shallow geological information. The self-induction electric field generated by a shut off of primary field is superimposed with the early secondary field, which causes the results that the detection result can't reflect the shallow stratum information and form a shallow blind area, and that the detection of early pure secondary field can fundamentally eliminate shallow blind spots. This paper first introduces the coil device of the opposing coils transient electromagnetic method, illustrates the method of the early secondary field acquisition, and meanwhile briefly describes the zero flux surface and the analysis of the regularity of induced voltage. Taking the karst exploration of highway slope of a hydropower station as the research background, the authors obtained the apparent resistivity information of strata of different depths, and delineated the spatial distribution of karst based on the analysis of the inversion results of the test data. It is proved that the opposing coils transient electromagnetic method can effectively avoid the impact of primary field shutdown on the secondary field, basically eliminates the shallow blind zone, and realizes the accurate exploration of the shallow stratum.