Abstract: For electromagnetic wave-absorbing materials, maximizing absorption at a specific frequency has been constantly achieved, but enhancing the absorption properties in the entire band remains a challenge. In this work, a 3D porous pyrolytic carbon (PyC) foam matrix was synthesized by a template method. Amorphous carbon nanotubes (CNTs) were then in-situ grown on the matrix surface to obtain ultralight CNTs/PyC foam. These in-situ grown amorphous CNTs were distributed uniformly and controlled by the catalytic growth time and can enhance the interface polarization and conduction loss of composites. When the electromagnetic wave enters the internal holes, the electromagnetic energy can be completely attenuated under the combined action of polarization, conductivity loss, and multiple reflections. The ultralight CNTs/PyC foam had a density of 22.0 mg·cm⁻³ and a reflection coefficient lower than −13.3 dB in the whole X-band (8.2–12.4 GHz), which is better than the conventional standard of effective absorption bandwidth (≤−10 dB). The results provide ideas for researching ultralight and strong electromagnetic wave absorbing materials in the X-band.