Abstract: Numerical simulations of electromagnetic field of 0.1-0.1T Hz in different circumstance are carried out with 3D-FDTD (three-dimensional finite-difference time-domain). Electric dipole field in the whole conductive space of sea is presented and electromagnetic response of the seabed geological model of low impedance is simulated taking Gauss impulse as current source. Then, transient electromagnetic response of water-filled cracks abnormal in the tunnel to loop-source is analyzed. UWB (Ultra wideband) through-wall detection model is simulated and calculated, and the simulation results of reinforced concrete detection with UWB electromagnetic wave and narrow-band electromagnetic wave are analyzed and compared. More than that, activation of TE6, 2 in line-joint cavity of 94 GHz gyrotron oscillator is modeled with MAGIC, and the problems existing in the simulation are presented. Finally, we give a deep analysis on the parallel computing of FDTD program and the idea to develop gyrotron three-dimensional transient field simulation software by using FDTD.