The generation of high energetic proton beams and propagation characteristics in laser-plasma interaction is simulated by two-dimensional relativistic electromagnetic particle-in-cell simulation code (2D-PIC). The results show that the density gradient scale length thickness is crucial to the proton energy spectrum. The quasi- static electric field becomes stronger as increasing the density gradient scale length, which results in the enhancement of the maximum proton energy. Density gradient scale length affects high- energy proton beam emitted from thin target by certain constraints. The collimation and the beam divergence of the accelerated protons are expected to strongly depend on the spatial coupling between the laser pulse and the target plasma.