A physical design scheme of final optical assembly (FOA) to achieve 351 nm propagation with high fluence is introduced. The method of optimization design is researched and the experimental study is completed on No.9 system of SG-Ⅱ laser facility. 33 shots have been fired with beam diameter of 310 mm, laser pulse width of 3 ns and 1053 nm energy from 1000 to 4500 J. In the process of the experiment, the most efficiency of third harmonic conversion is 69.6% and the maximum fluence of 351 nm propagation is 3.76 J/cm2. And emphasis is given on the number of contamination change inside the FOA and the process of the optical damage originating from the high fluence propagation. The results show that the first cause to the optics damage with 3 J/cm2 at 351 nm is induced by amounts of contaminants which come from the scatter laser irradiating to materials.