Abstract: In this paper, the detection depth of transient electromagnetic(TEM) field method is described in several aspects. The “smoke ring effect” of the equivalent electric current ring downward extension depth was cited. The diffusion depth of the maximum plane wave field was derived. And through the analysis of the time-frequency(T-F) the relation and distinction between the penetration depth of time domain and frequency domain was explained. Ground actual detection time needed for the same depth of information is at least twice as much as one-way diffusion depth, namely double travel time. According to several aspects of electromagnetic induction, reflection of electromagnetic waves, the frequency sounding curve of false phenomenon of extreme value and quasi seismic converted wave slowness, etc., this conclusion is discussed.