The work aims to rapidly prepare gram-scale, high-quality two-dimensional magnetic Fe5GeTe2 single crystals and develop new absorbing materials with van der Waals structure, room-temperature ferromagnetism, and excellent microwave absorption performance. In this work, Fe5GeTe2 single crystal samples were synthesized by adjusting the content of transport agents and shortening the constant temperature period of chemical vapor transport method. Fe5GeTe2 single crystals were mixed with paraffin wax according to different mass fractions. Their microwave absorption performance was analyzed by a coaxial line test. Gram-scale, high-quality Fe5GeTe2 single crystal samples were prepared successfully. The microwave absorption performance of Fe5GeTe2 single crystals was analyzed by the coaxial wire method with a vector network analyzer. The results indicated that the absorption performance of 30% Fe5GeTe2 was poor, and the minimum reflection loss value was only −4.8 dB. However, as the mass fraction of Fe5GeTe2 increased to 70%, the enhanced Fe5GeTe2 load in the Fe5GeTe2/paraffin composites led to improved interface polarization and enhanced impedance matching. The electromagnetic wave could smoothly penetrate the Fe5GeTe2/paraffin composites and then be dissipated via dielectric loss and magnetic loss, significantly enhancing its absorption performance. The minimum reflection loss value could reach −35.5 dB. The successful synthesis of gram-scale high-quality Fe5GeTe2 single crystals establishes a solid material foundation for researching the fundamental physical properties of this two-dimensional ferromagnet. Additionally, Fe5GeTe2 with room-temperature magnetic properties is not only a new type of absorbing material with excellent performance, but also provides a new choice for the research of composite absorbing agents.