The invention discloses a seepage prevention fertilizer synergist, which is made from raw materials with parts by weight: luffa powder 7-9, spent mushroom compost of lentinus edodes 22-25, sugar beet pulps 4-5, kanuma soil 14-16, potassium nitrate 6-7, vermiculite 10-12, cottonseed cakes 18-22, polyaspartic acids 2-4, earthworms 6-7, fresh pig urine 85-95, papermaking black liquid 9-10, embelia laeta fresh leaves 8-10, effective microorganisms (EM) complex microbial inoculants 2-4 and moderate water. The seepage prevention fertilizer synergist can excellently improve property of used soils by utilizing the kanuma soil, the vermiculite and various organic compounds, improves structure ratio of mass particles, optimizes absorption environment of crop roots, fixes fertilizers on the crop roots through excellent adsorbability, reduces seepage of water and fertilizer, prevents soils from hardening, reduces production cost of crops, and reduces pollution of the fertilizers to underground water.