The inertia fluid model proposed by Yoshida et al. can only interpret the influence of ambient pressure on the average size of nanoparticles prepared by pulsed laser ablation. Basing on the model, the Maxwell velocity distribution of the initial ablated particles is considered, a new analytic expression of the size-distribution of nanoparticles is obtained. The simulation results are consistent with the statistic data from Yoshidas experiments under different He pressures. Additionally, the size-distribution of nanoparticles is simulated using the modified model under different ambient gases (He, Ne and Ar),the simulation results coincide with experimental data. The conclusions may serve as the basis for realizing the uniformity and controllability of Si nanoparticles.