It is essential to manufacture microwave absorbers with strong absorption as well as tunable absorption bands at a low filler content. However, it remains challenging for pure biomass material to reach this goal without loading other components. MoSe₂, as a transition metal chalcogenide with semiconductor properties, has emerged as a potential microwave absorber filler. Herein, bacterial cellulose (BC)-derived carbon nanofibers/MoSe₂ nanocomposite was fabricated and phosphoric acid was used to dope phosphorus in BC, in which MoSe₂ microspheres were dropped on the BC network like a dew-covered spider web. This unique network structure enhances conductive loss and multiple reflections of the incident wave. The collocation of BC and MoSe₂ is helpful to impedance match and introduces interfacial/dipolar polarization loss; moreover, the P-doping of BC helps to tune the absorption bands. Overall, the optimal reflection loss of undoped one reaches −53.33 dB with only 20 wt.% filler content, whose main absorption peaks focus on X-band. Interestingly, after the P-doping of BC, the main absorption peaks move to Ku-band and the optimal reflection loss gets stronger (−66.84 dB) with the same filler loading. Strong absorption and tunable absorption bands can be realized, and thus wide frequency range is covered. This work is expected to enlighten future exploration of biomass carbon materials on high-performance microwave absorption materials.