The influence of lignin's primary oxygen-containing functional groups, phenolic OH (Ph-OH) and methoxyl (OMe) groups, especially the Ph-OH/OMe ratio, on its UV absorptivity and long-lasting UV blocking remains unclear. In this study, organosolv lignins (OLs) with varying Ph-OH and OMe contents were prepared to evaluate their UV absorptivity and photostability by EPR (electron paramagnetic resonance). As the Ph-OH contents increased from 0.32 to 6.09 mmol/g, lignin with a higher Ph-OH content displayed better absorptivity with a strong correlation (Pearson's r = 0.95). However, an excessive Ph-OH content (>5.5 mmol/g) reduced the spin content of lignin, negatively affecting its photostability. Taking Ph-OH and OMe groups as the cofactor, OLs with Ph-OH/OMe ratios between 0.44 and 0.64 showed more favorable UV-blocking performance both in the initial state and after UV irradiation. Moreover, lignin-based sunscreens exhibited increasing SPF value even after 8 h of UV exposure, confirming lignin's long-lasting UV-blocking property.