The invention relates to a liquid fertilizer used for eggplant based on biogas slurry and its application method, which comprises the following steps by weight: 70-90 parts of biogas slurry, 10-15 parts of ammonium nitrate, 3-5 parts of dipotassium hydrogen phosphate, 5-8 parts of potassium chloride, 1-3 parts of calcium nitrate, 0.8-1.5 parts of ferrous sulphate, 0.5-0.8 parts of magnesium chloride, 0.2-0.5 parts of zinc sulfate, 0.5-1.5 parts of boric acid, 0.3-0.5 parts of manganese sulfate and 0.2-0.5 parts of ammonium molybdate. According to the invention, the biogas slurry which is obtained by anaerobic fermentation of livestock and poultry excrement and crop straw through a methane tank, the nutrient composition required by the growth of the eggplant can be scientifically added to prepare the liquid fertilizer. The liquid fertilizer has the advantages of comprehensive nutrition, sufficient raw material source, low cost and safe environmental protection. The liquid fertilizer is diluted by using clear water, and is taken as an eggplant foliar fertilizer for spraying or a nutrient solution for soilless culture, and can obviously improve the growth property of the eggplant, the incidence of disease is reduced, and the output is increased to more than 14%. The liquid fertilizer solves the problems of difficult application, difficult transportation and environmental pollution due to secondary discharge.