In the investigation of Inertial Confinement Fusion (ICF), the high power laser beam must be focused on the target (focus plane) given. In the ICF indirect drivers, the target fixed in a cavity is cylinderal, there are the Deuterium and the Tritium in the target, the experiment result relatives with the temperature radiated in the cavity, and the temperature relatives with the laser energy power incided. Thus, it is key that there must be enough of energy incided, and the energy runs out barely. So, to increase the laser energy incided, the focus spot should be very small. Base on the design of focus optical systems of the SHENGUANG serial facilities and the theory analyses, as the diffraction of the light, the laser beam passing through the optical systems gets a spot on the focus plane. In order to reduce the edge of the main spot or the focus spot size, we advance a method of dividing aperture of the focusing optical system, the diffraction theory of light shows that the diffraction main spot is reduced, it indicates that the method of dividing the aperture may be recommended.