To study damage effectiveness of strong electro-magnetic pulse to components of equipments, the power density in area of MOS circuit, diodes and transistor of a computer is simulated, using the method of the finite-difference time-domain (FDTD). Coupling laws in different areas are achieved, and then judging the damage efficiency of components. Electromagnetic pulse reflects constantly in computer box, causing power density appears oscillations. Energy gradually declines to zero, for it radiates outward from slots. Field concentration around PCB board results in dissociation of field strength, and slows down the attenuation of energy. Finally, formula of power density at random field strength and rise time is also obtained.