Abstract: Polythioether is a kind of sulfur-containing polymer with C-S bond as the main chain, which possesses a high refractive index and light transmittance, but exhibits weak ultraviolet (UV) resistance and requires strict synthesis conditions required. In this manuscript, thiocyanate-initiated anionic ring-opening polymerization of urethano-substituted episulfide was carried out in an open vessel with a polar solvent to prepare functionalized polythioether with high molecular weight (M_n>260×10³). The tert-butyloxycarbonyl (Boc) moiety of the urethano group on the side chain was deprotected and converted to a primary amine group, which was readily reacted with the aldehyde group of vanillin to form the large-sized Schiff base-conjugated side group. After modification, polythioether achieved enhanced functionality, the UV protection factor (UPF) reached up to 216.6, and the UV transmittance was low (T_UVA<1%, T_UVB<0.5%). Consequently, it possessed strong intrinsic UV resistance. Meanwhile, the modified polythioether film exhibited high visible light transmittance (>80%). Therefore, the introduction of phenolic hydroxyl, substituted phenyl, and large conjugated Schiff-base imine groups into the side chains of polythioethers not only effectively improved their UV absorption and shielding capabilities, but also maintained good transparency, making them suitable for developing optical coatings and thin film materials with excellent intrinsic UV-shielding properties and application potential.