Lightweight, flexible and anisotropic porous multiwalled carbon nanotube (MWCNT)/water-borne polyurethane (WPU) composites are assembled by a facile freeze-drying method. The composites contain extremely wide range of MWCNT mass ratios and show giant electromagnetic interference (EMI) shielding effectiveness (SE) which exceeds 50 or 20 dB in the X-band while the density is merely 126 or 20 mg cm^{−3}, respectively. The relevant specific SE is up to 1148 dB cm^3 g^{−1}, greater than those of other shielding materials ever reported. The ultrahigh EMI shielding performance is attributed to the conductivity of the cell walls caused by MWCNT content, the anisotropic porous structures, and the polarization between MWCNT and WPU matrix. In addition to the enhanced electrical properties, the composites also indicate enhanced mechanical properties compared with porous WPU and CNT architectures.