Abstract: In design of hohlraum targets, the design of laser beam inlet is one of the most important problems. Because the mattar on the edge of inlet is heated by laser when-laser is injected, the matter expands isothermally to the inside of the inlet and finally the mattar in the inlet inhibits the laser from coming into the inlet. This is the closure phenomenon. Since the numerical simulation of the closure is two or three dimesion calculation, it is usually very difficult.In this paper a simple physical modle of the plane inlet target has been developed to compute closure time and ratio of injected laser energy in a very large,range of laser parameters.The qualitative tendency and the experienced dependence of the injected laser energy ratio versus the radius of inlet and the laser parameters have been obtained on the basic of numerical computations.The experienced formula of the threshold radius of inlet versus laser parameters has also been compared with the values of experimental measurement of plane inlet targets. The injected ratio is agreed within the order of 25%.