The invention relates to a preparation method of a biological root-irrigation fertilizer special for a nursery-grown plant. A formula of the biological root-irrigation fertilizer includes the followings in percentage by weight: 1-2.1% of phenylalanine, 0.5-1.5% of vitamin D2, 0.8-1.8% of ferrous citrate, 1-2% of bifidobacterium bifidum, 0.1-1.1% of potassium chloride, 1.2-2.2% of a Chinese angelica extract, 1.6-2.6% of streptococcus faecium, 0.7-1.7% of mock strawberry, 1.3-2.3% of herb of ajuga ciliata, 0.9-1.9% of acinetobacter calcoaceticus, 0.2-1.2% of dandelion, 0.6-1.6% of aspergillus oryzae, 2.8-3.8% of menadione dimethylpyrimidinol bisulfite, 2.9-3.9% of bacillus lentus, 2.1-3.1% of manganese carbonate, 2.2-3% of snake gall extract, 2-3.2% of herba violae, 2.5-3.5% of corn oligopeptide powder, 2.2-3.1% of probiotics and 73.4-54.4% of water. The nursery-grown plant absorbs sufficient water well, the higher survival rate of the nursery-grown plant is guaranteed, rooting can be promoted to enable a root system to be prosperous, the survival rate of transplanting is increased, disease resistance is enhanced, leaves are invisible green, flowers are delicate and charming, and early aging and yellow leaves are prevented; and a soil aggregated structure is relieved, soil is prevented from being caked, soil sterilization is achieved, breathability is enhanced, and dead seedlingsare prevented.