This paper studies the thickness of the free limiting layer in the laser shock processing. It analyses the relationship among the high intensity pressure caused by coating vaporization and explosion and transmitted into the interior of the metal layer and that of the free limiting layer, the adherent force between the free limiting layer and the coating, and the properties of the free limiting layer. On the above basis, the optimum thickness of the free limiting layer is obtained, which guarantees not only the optimum LSP effect but also the elimination of the free limiting layer at a time.