Brown carbon (BrC) plays an important role in aerosol light absorption, yet its relative humidity (RH) dependent optical evolution remains poorly constrained. Based on field observations and ultra-high-resolution mass spectrometry, this study reveals that the light absorption of water-soluble BrC is nonlinearly regulated by the interplay between pollution level and RH. The mass absorption efficiency at 365 nm shows a biphasic response to RH. It increases with RH, reaching a maximum in the range of approximately 50%–60% RH, and then decreases when RH exceeds this threshold. Molecular-level analysis reveals that RH may alter the chemical fate of water-soluble BrC chromophores, steering their formation and attenuation pathways while concurrently reshaping their optical absorption profiles. In the low-to-moderate RH regime, liquid-phase chemical processing drives the accumulation of chromophores. Beyond the inflection point, increasing aerosol liquid water activates several synergistic suppression pathways, including oxidative bleaching, hydrolytic cleavage of N/S-containing chromophores, and physical dilution. High pollution levels may partially offset the RH-driven suppression effects. These findings challenge static parameterizations of BrC in models and highlight the necessity of incorporating RH-driven molecular restructuring into radiative forcing assessments.