Abstract: A preliminary understanding of electromagnetic pulse generated by nuclear explosions in space has been developed with case signal model. For observers in large distances, a net dipole can be resulted from the average movements of space charges. Based on the law of electron movements, the electric dipole moment of the whole system has been analytically described. The radiation fields are closely related to the derivatives of the dipole moment with time. The peak value of radiation signal in far field increases linearly with the initial kinetic energies of electrons, and quadratically with the radius of bomb and with the rise time constant. The peak is irrelevant to explosion yield, while the peak time will be postponed as the yield decreases.