Abstract: With the gradually increasing protection awareness about electromagnetic pollution, the demand for absorbing materials with renewability and environmental friendliness has attracted widespread attention. In this work, composites consisting of straw-derived biochar combined with NiCo alloy were successfully fabricated through high-temperature carbonization and subsequent hydrothermal reaction. The electromagnetic parameters of the porous biocarbon/NiCo composites can be effectively modified by altering their NiCo content, and their improved absorbing performance can be attributed to the synergy effect of magnetic–dielectric characteristics. An exceptional reflection loss of −27.0 dB at 2.2 mm thickness and an effective absorption bandwidth of 4.4 GHz (11.7–16.1 GHz) were achieved. These results revealed that the porous biocarbon/NiCo composites could be used as a new generation absorbing material because of their low density, light weight, excellent conductivity, and strong absorption.