An ultra-light 2D-3D heterojunction composite is successfully designed and fabricated by introducing biomass-based carbonaceous aerogel as the skeleton structure and embedded 2D BiPO4 nanorods. The biomass-based carbonaceous aerogel is not only provided a large and continuous surface with porous for photocatalytic nanoparticles, but also supported the synergistic enhancement between substrates and BiPO4 nanorods, which enhanced the electron - hole separation and the solar energy collecting efficiency. In consequence, the BiPO4/biomass-based carbonaceous aerogel (BP/BCA) composite showed the outstanding activity for degradation of simulated dye (Methylene blue, MB) under visible light irradiation, which was almost 7.12 times as high as that of pure BiPO4. Furthermore, the improved photocatalytic activity was achieved attribute to the efficient production of catalytically active species (O-2(center dot-) and( center dot)OH), which were demonstrated by means of extensive mechanism research. The ultra-light 2D-3D heterojunction composite offers opportunities for future energy-efficient photocatalyst in this photocatalytic system.