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The properties of third harmonic beam splitters for high power laser system were studied in this paper. The third harmonic beam splitter, which has high transmittance at 1053nm and 527nm, and high reflectance at 351nm, was designed using regular quarter-wavelength stacks. Considering the optical properties, electric field distributions and experimental results, we concluded the film stack of Glass/(HL) 12 3H2M2L/Air) was a better option for obtaining a third harmonic beam splitter with high laser induced damage threshold (LIDT). In this study, electron beam evaporation was employed to fabricate these beam splitters on fused silica substrates. The experimental results showed that the samples had good optical performance. The LIDT at 355nm is about 10J/cm 2 (355nm, 7ns). To understand the influence of post-treatment on the LIDT of the beam splitter, the as-deposited coatings were treated in vacuum chamber with oxygen plasma and annealed in clean ambient air, respectively. The result exhibited the LIDT at 355nm was increased with oxygen plasma treatment, while decreased after annealing in clean ambient air.