Abstract: Agricultural straw (such as reed straw) has been underutilized to result in wasted resources and environmental pollution in recent years. A novel biodegradable packaging material, mycelium composite material can be expected to replace the foamed plastics. Fungi strains are also used to degrade agricultural straw, in order to improve the utilization rate of agricultural straw. This study aims to optimize the preparation process of mycelium composite materials with high compressive properties. A systematic investigation was implemented on the effects of fungi strains, inoculum amounts, substrate types, and particle sizes on the compressive property of mycelium composite material using compressive strength as the evaluation index. A total of ten fungi strains were selected, including Ganoderma resinaceum, Pleurotus ostreatus, Pleurotus sajor-caju, Trametes sanguinea, Pleurotus geesteranus, Flammulina filiformis, Agrocybe aegerita, Pholiota squamosa, Lentinus edodes, and Hericium erinaceus. The fungi strains were inoculated into kenaf and cottonseed hull compound substrate, in order to cultivate mycelium composite material. Results showed that the compressive strength of Ganoderma resinaceum group was significantly higher than the rest strains, due mainly to the compact external mycelial layer and tight hyphal network. The high growth rate was also attributed to a more compact mycelium network structure. Ganoderma resinaceum was inoculated into 17 substrates, including noil, kenaf, reed straw, cottonseed hull, corn cob, sawdust, and eleven compound substrates composed of the above six raw materials. The best compressive strength was achieved in the RCC group (reed straw and corn cob). The reason was that the reed straw and corn cob were provided the appropriate void structure and sufficient nutrients, which was conducive to the dense hyphal network. Then Ganoderma resinaceum was inoculated into RCC for the particle sizes and inoculum amounts. The highest compressive strength was achieved in the substrate with a wide range of particle sizes (0.075-2.000 mm). The substrate was combined with the large and small particle sizes, in order to provide the suitable nutrition and growth space, thus promoting the formation of a dense network structure of mycelia. The highest compressive strength was achieved in the 7.5% of inoculum amount. The mycelium colonization was weak in the case of low inoculum amount, thus resulting in the lower compressive strength of the material. Meanwhile, the excessive inoculation amount also caused the mycelia to compete for nutrients and space, thus reducing the density of the mycelium network structure for the low compressive strength of the material. An optimal combination of parameters was then achieved to produce the mycelium composite material: Ganoderma resinaceum as the tested strain, reed straw, and corn cob as the compound matrix, 0.075-2.000 mm of particle size, and 7.5% of inoculum amount. The high compressive strength and excellent buffering performance were obtained under the conditions. The compressive strength of the mycelium composite material reached 190.99 kPa at 10% relative deformation, which was 1.9 times that of 18.0 kg/m³ EPS. The findings can provide a strong reference to optimize the parameters and compressive properties of mycelium composite material. The positive significance was offered to improve the utilization of agricultural by-products, such as reed straw.