NOVELTY - Preparing methomyl insecticide comprises e.g. adding methylthio acetaldoxime in water and pulping, dropping methyl isocyanate in methyl acetaldehyde oxime slurry and reacting, controlling the reaction temperature, adding activated carbon in methomyl mixed solution, raising the temperature to dissolve the material, micro-pore filtering, placing in a crystallization kettle, cooling and crystallizing, separating out crystal, controlling stirring and cooling speed, completing crystallization, cooling, filtering, adding methomyl mixed solution in a double-cone dryer, cooling and packaging.    USE - The method is useful for preparing methomyl insecticide (claimed).    ADVANTAGE - The method is convenient to operate and controllable, is safe, reduces the byproduct production, improves the yield and quality, reduces the energy consumption and the production cost, improves the production field environment, and reduces the generation of three wastes.    DETAILED DESCRIPTION - Preparing methomyl insecticide, comprises adding methylthio acetaldoxime in water and pulping, dropping methyl isocyanate in the methyl acetaldehyde oxime slurry and reacting, where the mass ratio of methylthio acetaldoxime and methyl isocyanate is 1:1.0-1.3, controlling the reaction temperature to be 5-15 degrees C, where the reaction time is 1.5-3 hours, after the reaction is finished, adding activated carbon in methomyl mixed solution, raising the temperature to 50-70 degrees C to completely dissolve the material, micro-pore filtering, then placing into a crystallization kettle, cooling and crystallizing, where the crystallization temperature is 65-70 degrees C and reducing the temperature to 50-55 degrees C within 45-90 seconds, separating out crystal, controlling the stirring speed and cooling speed and maintaining each 30 minutes to 1 degrees C, cooling to 40-45 degrees C, completing crystallization, cooling to 15-25 degrees C for 15-30 seconds, filtering, adding the filtered methomyl mixed solution into a double-cone dryer and drying for 3-4 hours until the moisture content is less than 0.3%, where the methomyl weight ratio is more than 98.5%, cooling obtained residue to a temperature of less than 40 degrees C and packaging.