The invention relates to a fungus-based biomass thermal-insulation material with corn straw used as a main material and a preparation method of the material. The material is prepared from, in parts by mass, 60-85 parts of corn straw, 5-20 parts of rice bran and 10-20 parts of a biomass thermal-insulation material additive. The preparation method comprises preparation of a substrate, sterilization treatment and inoculation culture. The corn straw and other biomass materials are taken as the raw materials, the characteristics that fungal hyphae grow quickly and hypha bodies have high twisting force are used, and the corn straw and other biomass materials are converted into the environment-friendly biomass thermal-insulation material. As an environment-friendly biomass thermal-insulation material with market prospect and potential, the biomass thermal-insulation material is simple to prepare, low in production cost, excellent in thermal-insulation performance, low in density and high in strength.