The work aims to prepare a magnetic carbon-based composite with broadband microwave absorption properties by using biomass as carbon and adding sodium chloride as active agent. Magnetic carbon-based microwave absorption composite was successfully prepared by one-step pyrolysis method, and its phase, morphology and magnetic property were characterized by XRD, SEM, BET and SQUID. Then, the electromagnetic parameters were tested by VNA. The composites with different components all had broadband microwave absorption properties, and the microwave absorption properties were insensitive to the dosage of biomass in a certain range. When the dosages of biomass were 0.8, 1.0, 1.5 and 2.0 g, the different magnetic biomass carbon-based composites showed the effective absorption bandwidth (EABW) of 5.7, 6.2, 5.8 and 5.9 GHz under 30 wt.% filler loading. The insensitivity of microwave absorption properties to dosages of biomass can avoid the impact of biomass on the microwave absorption properties due to its different origins and growth cycles. In addition, it is beneficial to practical operation of preparation process, and improves the stability of the microwave absorption properties.