Abstract: Low cost, high performance, and good environmental stability are the key factors to determine the application of microwave absorbent. In this study, the wasted platanus tree fruits were taken as raw biomass materials, which were combined with the kaolin rock, one kind of abandoned coal mine resources, to construct the dual wastes-derived composite for microwave absorption. The obtained carbon microtubes/kaolin rock composite was optimized by controlling their interfacial interaction followed by high-temperature pyrolysis to reach efficient absorbing capability towards microwave radiation. The experimental results show that the acid-modified carbon microtubes (CMT-ac) and the alkali-decorated kaolin rock (KR-al) combined well to supply a large number of heterogeneous interfaces to strengthen the interfacial polarization mechanism. As a result, their conductivity difference under the irradiation of electromagnetic wave enabled greatly attenuating electromagnetic wave. The final KR-al@CMT-ac sample achieved an effective absorption bandwidth of 6.3 GHz (11.7~18.0 GHz) at a matching thickness of only 2.0 mm and a minimum reflection loss of −51.5 dB at 8.08 GHz at a thickness of 3.0 mm. The improvement in microwave absorption performance is due to the enhanced interface polarization and conduction loss. This study will provide an effective strategy for the design of low-cost and high-performance dielectric absorbents.