The invention discloses a fertilizer additive. The additive comprises, by weight, 5-10 parts of chitosan, 7-13 parts of humic acid, 10-20 parts of modified montmorillonite, 6-11 parts of sepiolite powder, 3-8 parts of coal gangue, 4-6 parts of potassium fulvic acid, 1-3 parts of litsea elongate leaves, 2-4 parts of cuscuta chinensis, 2-4 parts of dried pueraria lobata branches and leaves, 1-2 parts of dried wild chrysanthemum flowers, 2-3 parts of maggot powder, 30-40 parts of soybean bran, 4-9 parts of seaweed powder, 2-7 parts of borax and 2-4 parts of water-retaining agent. According to thefertilizer additive, the environment is protected, the soil is improved, the effective components of fertilizer are increased, the slow release effect is remarkable, the utilization rate of the fertilizer is increased, and the fertilizer additive is an environment-friendly fertilizer additive suitable for various kinds of chemical fertilizer.