The invention discloses a preparation method of biogas slurry and biogas slurry-based agriculture and forestry cultivation growth essence. The preparation method comprises the following steps: addingan ER composite microbial agent into a mixture of a livestock and poultry excrement liquid and crushed straws, performing sealed fermentation, carrying out solid-liquid separation to realize the separation of biogas residues and biogas slurry to obtain the biogas slurry required by preparation of the agriculture and forestry cultivation and growth essence, and mixing the biogas slurry with urea, monopotassium phosphate, magnesium sulfate heptahydrate, ferric trichloride hexahydrate, zinc sulfate heptahydrate, copper sulfide monohydrate and cobalt chloride tetrahydrate according to a certain weight part ratio to prepare the agriculture and forestry cultivation and growth essence. The essence is widely applied, can be directly used as a nutrient solution for soilless culture and a liquid organic fertilizer for soil after being simply diluted with water without special preparation, and can effectively promote rapid growth of plants; and the preparation method is nontoxic, has no pollution, and solves the problems of waste and idleness, secondary pollution and disease breeding of agricultural wastes.