A direct three-dimensional (3-D) modeling of B-field in TDEM is proposed in the paper. We first introduce the forward modeling method of static magnetic field for the calculation of initial value of B-field. Then, we achieve the iterative calculation of B-field of magnetic media during the transient electromagnetic (TEM) stage by substituting the Cole–Cole susceptibility model into the B-field diffusion equation of double-curl induced magnetization intensity. The validity of this method is verified using several analytical solutions.