Different post-treatment processes involving the use of ammonia and hexamethyldisilazane (HMDS) were explored for application to 351 nm third harmonic generation ${\mathrm{SiO}}_2$ antireflective ($3\omega$${\mathrm{SiO}}_2$ AR) coatings for high power laser systems prepared by the sol-gel method. According to experimental analysis, the $3\omega$${\mathrm{SiO}}_2$ AR coatings that were successively post-treated with ammonia and HMDS at 150°C for 48 h and again heat-treated at 180°C for 2 h (N/H 150 180 AR) were relatively better. There were relatively fewer changes in the optical properties of the N/H 150 180 AR coating under a humid and polluted environment, and the increase in defect density was slow in high humidity environments. The laser-induced damage threshold of the N/H 150 180 AR coating reached $15.83\mathrm{J}/{\mathrm{cm}}^2$ (355 nm, 6.8 ns), a value that meets the basic requirements of high power laser systems.