The shaping effects for the intense laser pulse of a low-density plasma foil is investigated by numerical simulation employing a one dimensional particle-in-cell (PIC) code. Laser pulses with steep leading edge of different pulse durations are obtained by changing the thickness of the plasma foil.The results show that a moving-electron-layer (MEL) is generated and driven forward by the radiation pressure, the leading edge is reflected by the MEL and the main part of the pulse is transmitted when the MEL reaches the rear surface of the foil. The single MEL vanishes quickly without double-layer structure, so the peak power of the pulse has no obvious attenuation.