Highly efficient photon-to-electron conversion is crucial for achieving photocatalytic conversion. In this study, oxygen-doped carbon nitride nanocages (O@CNNCs) were engineered via dual strategies of morphology-controlled heteroatom doping, which was successfully used in the photocatalytic selective oxidation of xylose/xylan to xylonic acid. The nanocage-shaped O@CNNCs had a larger surface area, which was 4.02 times of carbon nitride (CN). Furthermore, with the assistance of morphology regulation and O-doping, O@CNNCs exhibit highly efficient photon-to-electron conversion, enhanced visible-light utilization, high photocurrent, low resistance, and fast separation/migration of electron-hole pairs. Correspondingly, the photocatalytic oxidation of xylose to xylonic acid using O@CNNCs was successfully achieved under mild reaction conditions with a yield of 83.4%. O@CNNCs have excellent recyclability, in which the yield of xylonic acid in the 5th cycle was 98.2% of its initial use. The O@CNNC photocatalytic system was also suitable for macromolecular xylan, and a xylonic acid yield of 77.34 mg was obtained when 100 mg xylan was used. The oxidation-active species captured experiments indicated that holes were crucial for the selective oxidation of xylose to xylonic acid. Overall, this study provides a new strategy for the preparation of photocatalysts with excellent photon-to-electron conversion and selective oxidation of biomass-derived feedstocks to xylonic acid.