In plasma source ion implantation (PSⅡ), the target immersed directly in a uniform Plasma is biased with high negative voltage pulse.Soaexpanding plasma sheath forms between the plasma and the surface of solid material and implant ions into the target. A numerical simulation model in one-dimensional planar geometry is developed to determine the temporal and spatial evolution of the sheath for the voltage pulse of rectangular and trapezoidal wave forms. Especially, the sheath evolution is simulated at the fall time of a trapezoidal voltage pulse.By numerical results the forcing of the presheath exhibits to be related to the pulse length and the applied voltage.