Abstract: This paper, from the detection principle, describes the close relationship between the artificial source frequency electromagnetic sounding and the transient electromagnetic sounding. It is two aspects of a problem and can be converted each other. Two kinds of detection methods are essentially the same, they have the same characters. But after all the problem is reflected from the frequency domain and time domain, the two domains have their own characteristics. The principle of two methods is based on Maxwell's equations, the forward formulas of two methods can be converted by Fourier transform, the actual detection signal characteristics of the sending and receiving, field region division and other major aspects, the similarities and differences between two kinds of detection methods are illustrated to deepen the understanding of detection theory and method. The two methods should be used optimally according to the reality.