In order to explain quantitatively the relation between the growth rate of Ag thin film and ambient gas pressure in the pulsed laser ablation, considering the dynamic process of the ablated particles in inert ambient and the deposition and resputtermg of thin films, supposing the probability of resputtering is proportional to kinetic energy of the ablated particles, the dynamic model of growing film is establish. The experiment result of T.Scharf et al. is explained quantitatively by considering the big grains.