To alleviate the energy consumption and CO₂ emission caused by the operation of electrical equipment in hot weather, passive radiative cooling (PRC) has been considered as an effective strategy owing to the non-power input and non-pollution. In spite of exciting process, PRC materials are still limited by the poor optical selectivity in practical applications. Herein, inspired by the potential optical-thermal properties of the porous structure and composition of skeleton, ultra-thin hydrophobic sodium alginate (SA) cellulose bio-film was designed including precisely 3D porous structure like skeleton and ultra-long hydroxyapatite nanowires (HAP ULNWs) as self-assembled supporting frame. The obtained bio-film could reflect more than 95.59% of solar radiation only within ultra-thin scale (∼220 μm) as well as emit 96.73% of thermal radiation. As the proof, the bio-film was used for the outdoor radiative cooling and about 12℃ maximum cooling effect could be obtained during the daytime under high solar radiation power density (>1750 W/m²). For evaluating the energy-saving potential, a building model covered with PRC film was established and 48.67% of average electric energy consumption could be saved per year in China. The work presents an alternative strategy to enhance the optical selectivity and may facilitate the high-performance cooling energy saving.