Abstract: The influence of ramp effect in transient electromagnetic was study by Fourier transform, the influencing mechanism of ramp effect was revealed that the EMF was changed from the derivative of B excited by step current in the ideal case to time, to the difference of B to time(related to turn-off time), its influence depends on the distinction between derivative and difference. On this basis, the influencing factors of ramp effect, such as the turn-off time, the resistivity and the size of the transmitting loop, were studied. The results showed that the amplitude spectrum of ramp current was lower than step current in the frequency domain, so the ramp effect in the time domain was shown as reducing the EMF, its influence was proportional to the turn-off time and resistivity, and inversely proportional to the transmitting loop size.