It remains a great challenge for magnetic/carbon-based microwave absorbers to achieve both strong and broadband absorption, especially at ultra-low filling ratio. Herein, nickel nanoparticles supported on porous silica/carbon composite (Ni@SiO₂/C) derived from POSS was prepared by a facile method. The carbonized Ni@SiO₂/C composite offers the following advantages: i) the three-dimensionally porous POSS-derived SiO₂/C skeleton provides electron transport paths and enhances conductive dissipation; ii) the porous magnetic/carbon composite provides interfacial polarization to enhance dielectric loss and forms strong magnetic coupling network to enhance magnetic loss; iii) multiple reflections and scattering between the porous framework enhance electromagnetic waves attenuation. Notably, the optimized Ni₁₅@SiO₂/C₇₅₀ achieves high reflection loss of −58.5 dB, and broad effect absorption bandwidth of 8 GHz with a thickness of 2 mm at an ultra-low filling ratio of 3 wt%. Besides, the Ni₁₅@SiO₂/C₇₅₀ has multiple properties of infrared stealth, flame retardancy, and water resistance.