Lightweight and chemically stable carbon are widely applied as attractive microwave absorption materials (MAMs). However, the effective response bandwidth of pure carbonaceous MAMs is limited due to the imbalance between conductivity and polarization loss. Herein, carbon with a large number of amorphous/nanocrystalline uneven phase interfaces prepared from commercial phenol–formaldehyde resin (PF) through simple anoxic carbonization exhibited excellent microwave absorption performance by balancing conductivity and polarization loss. Benefiting from the modification of the carbon nanocrystalline, a suitable electrical conductivity is obtained with a large number of amorphous/nanocrystalline uneven phase interfaces, allowing more incident microwaves to be lost. Thus, PF-650 exhibits a strong reflection loss of − 59.62 dB and a broadband effective microwave absorption of 6.32 GHz at 2.35 mm. In contrast to typical carbonaceous MAMs with multiple chemical compositions and complicated microstructures, this work provides a promising approach to the preparation of highly efficient and yielding carbonaceous materials for practical applications.