In order to realize less than 30μm the pointing error of the main laser and the large focal spot irradiation uniform on target surface better than 10% of the additional beam in ＂Shenguang- Ⅱ ＂ laser facility, a physical design of final optics assembly（FOA） has been proposed. Using finite element method, the static and dynamic states about the main machine components of FOA and the integral ICF target chamber are analyzed and the design of FOA is optimized. And using mathematics simulation, lens array has been analyzed and designed based on the focus lens about unit number, radius, thickness, long axis and short axis. Experimental results show that the pointing error of main laser on the target is 28.9μm , the large focal spot is 1000μm× 500 μm and the large focal spot irradiation uniform is 12.0%.