The influence of the density of initial ablated-particle on the spatial density distribution of the particles (Si and He) and the velocity distribution of the Si-particles was investigated via Monte Carlo simulation. With the increase of the initial ablated-particle density, both the maximum distance and the minimum distance after rebound of the mixed region from the target are decreased, and the width of the velocity distribution is increased. The splitting of the velocity was observed when the initial ablated-particle density was greater than 8.3×1025 m-3. These results give a good reference for further study of the Si nanoparticle nucleation.