Abstract: In order to receive the information of intense X-ray source inside cavity target with a size of a few hundreds micrometers, an aperture with ressonable scale is often designed in the wall of the cavity. X rays from inside rapidly heat the material of pinholerin, producing an ablation cloud that impedes transmission of the radiation signal viewed from the outside. So the transmission of intense radiation signal through pinhole is affected by the material and size of the pinhole, and depends on the parameters, for example, equivalent blackbody temperature and pulse shape etc. An analytical model which provides insight into the physics of closure process, is developed to describe the plasma closure effect.