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.