The development of a cost-effective process for biomass conversion into sugar platforms is a cornerstone of sustainable biorefining. Here, we report a Lewis acid-controlled catalytic strategy for nonenzymatic sugar production from hemicellulose in biomass using a highly active, magnetically separable core–shell catalyst. The catalyst comprises a Fe3O4 magnetic core, an acid-resistant silica interlayer, and a phosphorus (P)-doped porous carbon shell. Under mild hydrothermal conditions, the optimized catalyst (MC600P1.2) achieved a xylose yield of 86.9% from xylan (100 °C for 2 h) and 60.3–91.0% from diverse biomass feedstocks (corncob, corn stover, poplar, and bamboo, 160 °C for 2 h), outperforming previously reported systems. Such remarkable catalytic activity is attributed to the unique structural design of the catalyst. The silica interlayer acts as a protective shield of Fe3O4 core and bonds with the carbon layer to form a silica–carbon shell that provides an ideal scaffold. Meanwhile, P doping introduces defects and hydrogen bonds, enhancing the active site accessibility. The formation of thermally stable POx species (C–P–O, C3–P═O, and C–O–P) on the carbon support acts as Lewis acid sites. C–P–O efficiently activates H2O to produce H+ for glycosidic bond cleavage and suppresses xylose degradation, ensuring a high yield. These features result in high catalytic stability and reusability, maintaining high xylose yields after three consecutive cycles. This work provides new research avenues to produce nonenzymatic sugar based on active, deactivation-resistant, and easily recoverable heterogeneous catalysts.