AbstractTaking central loop configuration excited by a step pulse on the ground as an example, we study the probing depth of electromagnetic sounding with analytical method, finite difference time domain and timefrequency methods. The aim is to provide a basis for field work design in order to reach the expected target. Analytical calculation proves that the transient electromagnetic field propagates in the earth at a limited speed, and numerical simulation exhibits reflection of the field under the quasi‐static condition. The results show that the probing depth in transient electromagnetic exploration is mainly determined by measuring time because the time‐domain electromagnetic field follows the law of causality. The initial propagation velocity of the transient electromagnetic field is not related to earth resistivity. Due to earth dispersion the front edge of the impulse becomes smoothed gradually and every frequency component has its velocity related with resistivity, and longer measuring time will be required for the same depth in lower resistivity stratum. The maximum probing depth is the distance over which an electromagnetic wave goes to and back from some depth in the earth in a given time. The minimum depth is limited by apparatus's performance. However, the anomalous bodies embedded in the upper subsurface may be detected at a late time. The relation between time and frequency can be obtained from the time‐frequency density spectrum of transient field signals.