Abstract: A new type of lightweight backfill material is obtained by the natural growth of fungi. The unconfined compressive strength tests, permeability tests, scanning of electron microscope, and CT scanning tests are conducted to study the physico-mechanical and micro properties of composite mycelial lightweight soil (MLS). The results show that the MLS is a kind of porous and lightweight backfill material, formed by mycelium winding around the substrate filler materials. The colonization degree of mycelium has an important influence on its compressive strength. The permeability coefficient of the MLS is close to that of the silty clay. The permeability decreases with the addition of aggregate content, but the decreasing rate is related to the degree of mycelial colonization. The addition of aggregate improves the strength of the MLS. The higher the degree of the mycelial colonization, the more notable the increase in the compressive strength under the same aggregate content. The ratio of the peak strength to the residual strength is between 1.15 and 1.22. An exponential function equation is established between the compressive strength and the aggregate content. The MLS has the advantages of simple production process, low energy consumption and no environmental pollution, and is expected to become a new type of lightweight backfill material.