Abstract: The parameters of laser have impact on the process of producing quasimonoenergetic proton beams which use a high power laser interacting with plasmas. The duration of the laser is an important parameter of them. A one dimension particle-in cell code LPIC++ is used to simulate the P-polarized laser interacting with the cold dense target. In the simulation, the ponderomotive force pushing the plasma layer and launching a shock wave are observed. The moving shock wave in the target traps ions and reflect them to as much as two times of the ion-acoustic velocity. It is also observed that if suitable laser duration is utilized, just as the duration matches the thickness of the target, the energy spread of the proton’s beam is about 30%. The quasimonoenergetic protons beam’s energy also can be