The invention relates to the field of lubricating oil, and particularly discloses a composition used for food-grade gear oil, a preparing method thereof, gear oil including the composition and a preparing method of the gear oil including the composition. The composition includes, by weight, 5-10 parts of a food-grade antioxidant, 10-25 parts of a food-grade antiwear agent, 0.5-1.5 parts of a food-grade metal deactivator, 1.0-2.5 parts of a food-grade antirust agent, 0.5-1.5 parts of a food-grade corrosion inhibitor, 0.2-1.0 part of a food-grade oiliness agent, 0.1-0.5 part of a food-grade anti-emulsification agent and 0.01-0.3 part of a food-grade defoamer, with the balance being food-grade base oil A and the total weight parts being 100 parts. The composition and the gear oil including the composition have excellent waterproofness, oxidation resistance, wear resistance, extreme pressure properties, micro pitting resistance, rust resistance, emulsification resistance and foam resistance, can effectively meet requirements on heavy duty, extreme pressure properties, wear resistance, emulsification resistance, rust resistance, micro pitting resistance, and the like of gear systems in a food processing process, are nontoxic and harmless and do not contaminate foods.