Abstract: This paper studies on the thickness of the free limiting layer in the laser shock processing(LSP).The optimum thickness of the free limiting layer was obtained, after the relations are researched among the shocking pressure by the coating vaporization and explosion and the adherent force between the workpiece and the free limiting layer and the behaviors of the free limiting layer.Namely, not only the optimal effect of the LSP is obtained,but also the objective to clean the free limiting layer only once safely and effectually can be realised,which is proved by the experiment.