In this paper, the finite difference method is used to develop the Fortran software MCHII. The physical process in which the electromagnetic signal is generated by the interaction of nuclear-explosion-induced Compton currents with the geomagnetic field is numerically simulated. The electromagnetic pulse waveforms below the burst point are investigated.The effects of the height of burst, yield and the time-dependence of ?-rays are calculated by using the MCHII code.The results agree well with those obtained by using the code CHAP.