The invention relates to a synthetic method of long-acting sulfonamide. The synthetic method comprises the following steps: (a) sequentially adding methyl
alcohol, caustic soda flakes and sulfachloropyridazine
sodium into a reaction kettle, stirring for reaction for 1-15 hours at the pressure of 0.1-1.0 MPa and the temperature of 50-160 DEG C to obtain a first
mixed solution, cooling the reaction kettle to a normal temperature, and depressurizing the reaction kettle to normal pressure, wherein the
mass ratio of the methyl
alcohol to the caustic soda flakes to the sulfachloropyridazine
sodium is (1-3): (0.1-0.8): 1; (b) evaporating the first
mixed solution to
dryness, adding first deionized water for
dissolution, adjusting the pH value to be 5.4-6.3 for product
crystal precipitation, and carrying out suction
filtration to obtain a crude product, wherein the ratio of the methyl
alcohol to the first deionized water is (2-3) g: (1.5-2) ml; (c) adding the crude product into a decoloration 3-mouth
bottle, adding second deionized water for
dissolution, adjusting the pH value to be 12-14, adding
activated carbon, stirring for 10-60 minutes at the temperature of 0-100 DEG C, filtering, adjusting the pH of the filtrate to be 5.4-6.3 for product
crystal precipitation, carrying out suction
filtration, and
drying, wherein the ratio of the methyl alcohol to the second deionized water to the
activated carbon is (2-3) g: (4-6) ml: (0.05-0.2) g. According to the synthetic method, the yield of the long-acting sulfonamide can be improved, and the cost is reduced.