Abstract: It is possible using ray tomography to explain the response data of transient electromagnetic (TEM) exploration when we use wavefield transformation technology.But the pulse shape broadening phenomena of transformed wave field with increasing the receiver offset distance and the earth conductivity reduce the space resolution of the interpretation significantly.In this paper, we obtained the expression of the electrical vector potential of the large loop antenna which is excited by the step current in uniform losses space.By this expression, we discussed the relationship of the response pulse width with the receiver offset distance and the media permittivity.By using that relationship and the distribution characteristic of the induction eddy current in the earth, we explained the pulse broadening phenomena.We find the pulse broadening phenomena in the fictitious loss media resulting from the space distribution characteristics of the induced eddy current in the earth.Because the propagation velocity of the eddy current is very slow than the light velocity, the broadening phenomena in the fictitious losses media is also very evident than that in the physical losses media such as free space.