Reactive oxygen species (ROSs) offer strong oxidation capability for water purification, but their practical application is hindered by low energy transfer efficiency and fast exciton recombination. Herein, we construct a dual-shell UCMPs@g-C3N4@MSEP architecture through an integrated energy cascade regulation strategy. This architecture incorporates upconversion microparticles (UCMPs) for broadband spectral harvesting, along with an intimately interfaced g-C3N4 interlayer that achieves exceptional energy transfer efficiency exceeding 69.5%, thereby boosting ROSs generation by 1.7-fold. The second shell consists of magnetic sepiolite (MSEP), significantly enhancing surface hydrophilicity. This composite degrades 98.2% of ofloxacin within 40 min in various aqueous matrices, representing a 2.6-fold improvement over conventional g-C3N4. Experimental and theoretical analyses jointly confirm that the dual-shell configuration suppresses charge-carrier recombination. Liquid chromatography-mass spectrometry (LC-MS) was used to trace degradation intermediates and pathways. Toxicity assessment further confirmed that the photocatalytic process posed lower environmental risk. The enhanced activity arises from extended near-infrared light absorption via UCMPs and improved exciton separation enabled by the dual-shell design. This study offers a viable and environmentally benign strategy for efficient solar-driven water purification.