The invention relates to a pesticide preparation and a preparation method thereof, in particular to a trifluralin emulsion in water and a preparation method thereof. The trifluralin emulsion in water comprises the following components in part by mass: 0.5 to 48 parts of trifluralin, 1 to 20 parts of solvent, 1 to 16 parts of emulsifier, 0.1 to 5 parts of thickening agent, 0 to 8 parts of antifreezing agent, 0.1 to 1.6 parts of defoaming agent, 0 to 3 parts of antimicrobial agent, a proper amount of pH value regulator, and the balance of deionized water. The trifluralin emulsion in water is a selective pre-emergence soil treatment agent, and is mainly absorbed through coleoptiles and hypocotyls of weeds. The trifluralin emulsion in water has the effect on gramineous and partial small-seed broadleaf weeds, and is applicable to cotton, soybeans, rape, peanuts, potatoes and the like. The trifluralin emulsion in water does not contain or rarely contains an organic solvent, so the manufacturing cost and use cost are reduced, and the toxicity produced to operators, storers and transporters, and users is reduced in the process of production, storage and transportation; and the trifluralin emulsion in water is a product which is environmental-friendly and has high safety.