As the predominant source of autochthonous organic carbon in shallow lakes, aquatic macrophyte litter undergoes decomposition that fundamentally regulates organic matter formation and ecosystem turnover—processes critical to the regional and global carbon equilibrium. While solar radiation is widely recognized as a ubiquitous driver of litter degradation in terrestrial ecosystem, the mechanistic pathways governing photodegradation within aquatic environments remain remarkably elusive. To bridge this knowledge gap, we orchestrated a dual-approach study combining field assays with microbial-inhibition microcosms, targeting three macrophyte species—Nymphoides peltata, Potamogeton malaianus, and Phragmites australis—selected for their contrasting traits. By manipulating light intensity and spectral composition, we tracked the dynamic transformations of litter chemical components to disentangle the relative contribution of solar radiation to aquatic litter decomposition. Results from our 95-day in situ experiment reveal that exposure to full-spectrum solar radiation in surface waters (5 cm) substantially accelerates litter decay, though the magnitude of this facilitation is highly species-specific—ranging from ~78% for Phragmites australis and ~80% for Nymphoides peltata to a striking ~291% for Potamogeton malaianus. Crucially, the extent of this photodegradation is governed by an interplay between solar intensity and spectral composition, where photomineralization rates exhibit a positive correlation with irradiance—identifying radiation intensity as the pivotal threshold determining whether the net decomposition effect is stimulatory or inhibitory. While blue light spectra drove the most significant mass loss in surface waters, this regulatory dominance attenuated significantly at greater depths (40 cm), where green light treatments even induced a negative effect on decomposition. Beyond mere mass loss, solar radiation effectively catalyzed the release of nutrients and the breakdown of recalcitrant components, notably lignin, with the efficiency of these processes remaining contingent upon the intrinsic quality of the litter. Collectively, these findings underscore the role of photodegradation in enhancing aquatic litter turnover, bridging a critical knowledge gap regarding underwater photomineralization and providing the mechanistic evidence necessary to re-evaluate the carbon source/sink dynamics of shallow lakes in a changing global climate.