In this paper, the sunlight resistance of vat-dyed polysulfonamide (PSA) fibers was systematically studied, with particular focus on the mechanisms of UV-induced degradation and the protective role of vat dyes. Vat-dyed PSA with Vat Red 31 (PSA/VR), Vat Blue 6 (PSA/VB), and Vat Violet 13 (PSA/VV), were evaluated for UV absorption, free radical formation, color stability, and mechanical properties. UV absorption analysis revealed that PSA/VR exhibited the highest integrated UV absorption (1657.3), followed by PSA/VB (1355.2) and PSA/VV (1126.2), with lightfastness ratings ranging from 4 (PSA/VR) to 5 (PSA/VV). Electron spin resonance (ESR) analysis confirmed that free radical generation followed the order: undyed PSA < PSA/VR < PSA/VV < PSA/VB, indicating a direct correlation with UV absorption. Colorimetric analysis showed that undyed PSA fibers darkened over time (ΔK/S = −1.43), whereas vat-dyed PSA fibers exhibited a photodarkening effect, with PSA/VV demonstrating the highest color retention (ΔK/S = −0.81). X-ray photoelectron spectroscopy (XPS) analysis revealed significant amide bond degradation in PSA/VR (CO–NH content reduction from 14.85% to 3.63%), whereas PSA/VV exhibited better stability (retaining 6.38%). The results indicate that vat dyes enhance the photostability of PSA fibers, with PSA/VV offering superior sunlight resistance, making it a promising candidate for applications requiring extended UV exposure.