The invention relates to the field of preparing fertilizers from biogas anaerobic fermentation products, in particular to a method of preparing a fertilizer from biogas slurry and biogas residues, thefertilizer prepared through the method and application of the fertilizer. The method comprises the steps that a biogas anaerobic fermentation product is subjected to solid-liquid separation to obtainthe biogas residues and the biogas slurry; the obtained biogas slurry is concentrated to obtain a concentrated solution; the biogas residues, the concentrated solution and a selectable nutrient conditioner are mixed to obtain a mixture; and the mixture is subjected to aerobic composting, and the composing product and a selectable auxiliary material are compounded to obtain the fertilizer. Throughthe method, high-value utilization of the biogas slurry is effectively realized, the problems in biogas slurry storage and transportation are solved, and the method is less in investment and easy tooperate. More importantly, by use of the fertilizer, not only can the germination rate of seeds be increased, but also the germination time of the seeds is effectively shortened, and the draught resistance and water-logging resistance of crops are remarkably improved.