The invention discloses a preparation method of a solid decomposing inoculant, which comprises the following steps: 1, manufacturing a solid fermentation device; 2, preparing a seed solution; step 3, preparing fermentation liquor; and 4, carrying out solid fermentation to obtain the solid decomposition agent. The invention has the following beneficial effects: 1, bacteria and fungi are mixed in the product, spores are not easy to germinate after the fungi are fermented, the bacteria content is relatively stable after the bacteria are fermented, and the shelf life is longer than that of common products; 2, fermentation functional strains are directionally screened according to main components such as cellulose, protein, starch and lignin in a fermentation material in a test, and the product has the function of decomposing the cellulose, the protein, the starch and the lignin and is wide in application range; 3, a special fermentation formula for solid is researched through tests, and the bacterium content in unit product is increased, so that the use amount of the product is reduced, and the decomposition cost is reduced.