The invention discloses a field application method of a tobacco root knot nematode disease chemical reagent. The method comprises the following steps: carrying out the furrow application when in land preparation before the transplanting, and carrying out the hole application when in transplanting, wherein the dosage for field having light or having no root knot nematode disease in previous years when in furrow application and hole application is as follows: 2 to 3 kg of granules for furrow application, 3 to 4 kg of granular chemical reagent for the field having moderate disease, and 4 to 6 kg of granular chemical reagent for the field having severe disease; when in application of the chemical reagent, the mixed application with alkaline pesticide and fertilizer is avoided, the chemical reagent is not applied in rainy and stormy days, and the chemical reagent needs to be comprehensively and uniformly applied. The method is simple and convenient to use and accurate in dosage, a critical period for prevention application is taken, the application technique and method are improved, the preventive effect of the reagent for preventing a target object is improved, the slow release action for adopting the chemical reagent granules to prevent the root knot nematode disease is good, the action durability is prolonged, the application efficiency of the reagent is greatly improved by adopting the furrow and hole application technology, and the method is suitable for preventing intractable root diseases of crops with high economic value.