Abstract: The harmonic field of electric dipole source on the uniform earth surface is converted into transient field by inverse Laplace transform,and the B_z(t)/t transient curve is calculated.The H_z(t) transient curves on the stratified earth surface are obtained from cosine transform calculated by broken line approximation.From the analysis of calculating results it can be seen that the greater depth explored with the smaller offset in the TEM sounding could be achieved.