A preparation method of sulfadoxine belongs to the field of sulfanilamide antimicrobial drug preparation. Cyclization reaction comprises the following steps of: firstly pouring a sodium methoxide solution into a reactive pan, then successively adding methanamide and methyl ethyl methoxymalonate, keeping warm, recovering methanol, cooling for crystallization, drying by centrifugation, discharging,and drying to obtain 5-methoxy-4,6-disodium dihydroxypyrimidine; Chlorination reaction comprises the following steps of: firstly putting phosphorus oxychloride into a reaction vessel for heating, adding 5-methoxy-4,6-disodium dihydroxypyrimidine into the reaction vessel to react, decompressing and recovering phosphorus oxychloride until the material is dry, cooling, adding trichloro ethylene withuniformly stirring, putting into a hydrolysis pan for hydrolyzation, collecting a trichloro ethylene layer after standing and delaminating, followed by a neutralization reaction, controlling pH value, washing, removing a water layer, recovering trichloro ethylene, and releasing crystals to obtain 5-methoxy-4,6-dichloropyrimidine. The preparation method provided by the invention can be used to guarantee the product purity, prolong the service life of equipment, avoid the damage to the environment and human body, reduce emission, and save energy, and accords with foreign pharmacopoeia standard requirements.