The casual discharge of dyes from industrial settings has seriously polluted global water systems. Owing to the abundance of biomass resources, preparing photocatalysts for photocatalytic degradation of dyes is significant; however, it still remains challenging. In this work, a cuprous oxide/copper oxide composite was interpenetrated onto carbon nanosheets of cellulose-based flexible carbon aerogels (Cu₂O/CuO@CA_x) via a simple freeze-drying-calcination method. The introduction of the carbon aerogel effectively prevents the aggregation of the cuprous oxide/copper oxide composite. In addition, Cu₂O/CuO@CA_0.2 has a larger specific surface area, stronger charge transfer capacity, and lower recombination rate of photogenerated carriers than copper oxide. Moreover, Cu₂O/CuO@CA_0.2 exhibited high photocatalytic activity in decomposing methylene blue, with a degradation rate reaching up to 99.09% in 60 min. The active oxidation species in the photocatalytic degradation process were systematically investigated by electron spin resonance characterization and poisoning experiments, among which singlet oxygen played a major role. In conclusion, this work provides an effective method for preparing photocatalysts using biomass resources in combination with different metal oxides. It also promotes the development of photocatalytic degradation of dyes.