NOVELTY - Method for preparing an organic fertilizer through mixed fermentation of biogas residues and biogas slurry in biogas engineering, involves preparing external compost environment, using residual biogas wastes and straws as raw materials of the compost, and chopping the straws, detecting a water content of the biogas residues and the straws, uniformly mixing and stirring, composting the mixture in the greenhouse, uniformly laying, adding water, performing artificial turning, fermenting, and air-drying the mature compost, crushing and packaging.    USE - The method is useful for preparing an organic fertilizer through mixed fermentation of biogas residues and biogas slurry in biogas engineering (claimed).    ADVANTAGE - The method is simple, convenient, practical, cost-effective, good in promotion effect for output and quality of crops, and quite high in development and utilization values.    DETAILED DESCRIPTION - Method for preparing an organic fertilizer through mixed fermentation of biogas residues and biogas slurry in biogas engineering, involves (a) preparing external compost environment at 20-40 degrees C, where inside of the greenhouse is sealed, and artificial turning is performed to ventilate and supply oxygen at temperature above 40 degrees C, (b) using residual biogas wastes and straws as raw materials of the compost, and chopping the straws of 0.1-1 (claimed) length, where the residual biogas wastes are pig manure and sludge mixture left in the biogas, (c) detecting a water content of the biogas residues and the straws, uniformly mixing and stirring the biogas residues and the straws, composting the mixture in the greenhouse, laying plastic foils on the ground of 1.5 m length, 1.5 m width and 1 m height, and uniformly laying the mixture on the plastic foil as a bottom compost, (d) adding water to regulate a water content of the compost mixture with respect to the water content of the biogas residues and the straws, where the initial moisture content is 70%, (e) performing artificial turning after a temperature of the compost rises to 48-52 degrees C for 1-3 days, manually turning once every 3-6 days, mixing to ensure ventilation for composting oxygen and adjusting the temperature drop of 5-10 degrees C, where the late compost temperature is gradually decreased to 20-30 degrees C, (f) fermenting for 38-42 days at 20-25 degrees C until the compost has a dark brown appearance and is loose in structure, and (g) performing post-treating, comprising air-drying the mature compost at 105 degrees C, crushing and packaging product.