In this paper, we present a one-dimensional simplified model for the processes of gaseous discharge and charging on the surfaces of KDP crystal for one-pulse process in Plasma-Electrode Pockels Cell (PEPC). The PEPCs are used as large-aperture optical switch in laser drivers for inertial-confinement fusion. The plasma electrodes of the switch are produced by high discharge current, while a high voltage pulse is applied across a thin KDP crystal plate through the helium discharge plasma. The evolvements of discharge current, charging voltage on KDP crystal and the switch efficiency of PEPC are simulated. The model is very useful for the design of PEPC and prediction the behavior of the optical switch.