Designing highly reliable and practical microwave absorbers is one of the most important research directions in the microwave absorbing field. Many absorbents suffer from concentration-sensitivity and environmental-sensitivity dilemmas in practical applications. Here, sea urchin-like aggregates of MnO₂ nanotubes were synthesized by a simple hydrothermal method, which exhibit an outstanding impedance matching characteristic. The composites based on sea urchin-like aggregates of MnO₂ nanotubes show excellent microwave absorption performance in a wide concentration domain from 20 wt.% to 70 wt.%, corresponding to electrical conductivities from 1.86 × 10⁻⁷ to 1.85 × 10⁻⁵ S/m. Such a wide concentration range of absorbent for excellent microwave absorption is mainly attributed to the beneficial impedance matching properties of sea urchin-like aggregates of hollow nanotubes. A competitive absorption bandwidth of 3.36 GHz is achieved at 1 mm thickness, which can be broadened to 13.4 GHz by structural design. This work shows a new scheme for designing reliable and practical microwave absorbers benefit from the wide absorbent concentration domain.