The invention discloses a method for preparing a seedling bed by using modified biogas residue fibers. The method comprises the steps of firstly, carrying out acetylation pretreatment on biogas residue fibers, then processing the pretreated biogas residue fibers together with a biomass adhesive, reinforcing filler, a curing agent and a demolding agent in a high-speed mixing process, and preparing the seedling bed containing the modified biogas residue fibers by means of compression molding. According to the method, biogas residue obtained by means of anaerobic fermentation is high in contents of lignin and cellulose having a crystal structure, is beneficial to acetylation modification and can effectively improve the fluidity of the biogas residue fibers, thus overcoming the defects that the traditional straw fibers are low in acetylation grafting rate and more in by-products; on the other hand, the biogas residue fibers contain higher content of nutrient elements such as nitrogen, phosphorus, potassium, calcium, magnesium and silicon, the discarded seedling bed containing the biogas residue fibers is easily decomposed by microorganisms, and the degraded products can be taken as fertilizer for promoting the growth of seedlings, so that the seedling bed is environmentally-friendly. Compared with a plastic seedling bed, the seedling bed product containing the biogas residue fibers is low in energy consumption, environmentally friendly, high in breathability and degradable, thus being easy to popularize.