In this paper, it has been studied that the thickness of the free confinement medium in the Laser Shock Processing. Then the relationships are analyzed between the peak pressure induced by laser shock and the properties of the free confinement medium. On the basis, the optimum thickness of the free confinement medium is obtained, which ensure not only the optimum LSP effect but also the elimination of the remains of free confinement medium at a time. In the experiments, a kind of materials of free confinement medium was invented successfully, which is applicable to laser shock processing. The estimation of the free confinement medium theoretical thickness is accorded with the result of the experimental.