With the increasingly serious electromagnetic pollution, “thin, wide, light and strong” new composite microwave absorbing materials have attracted extensive attention. However, the development and application of new composite microwave absorbing materials are hindered by comparatively complex fabrication process, weak machinability and difficulty in regulation of electromagnetic properties. Here, as a rich renewable resource with a three-dimensional mesh interconnection structure, bacterial cellulose (BC), whose surface is rich in hydroxyl groups for absorbing cobalt and iron ions, is used as a carbon source to prepare Co3Fe7 carbon nanofibers (Co3Fe7/CNF) aerogel by one-step carbon reduction method. The research shows that Co3Fe7/CNF aerogel has three-dimensional mesh interconnection structure, high porosity, ultra-low density and high microwave absorption performance. When the carbonization temperature is 900 ℃, the reflection loss (RL) of Co3Fe7/CNF aerogel and paraffin mixture with low load (~3wt%) and thin thickness (3.5 mm) is -47.5 dB, indicating that BC biomass materials show great potential in “thin, wide, light and strong” microwave absorbing materials.