The protozoacide Diclazuril was synthesized by substituting 3,4,5-trichloronitrobenzene with 4-chlorophenylacetonitrile in the presence of NaOH, reducing with Fe/NH4Cl, diazotizing with NaNO2/HCl and adding with Et cyanoacetylcarbamate to obtain Et [2-[4-(4-chlorophenylcyanomethyl)-3,5-dichlorophenylhydrazo]-2-cyanoacetyl]carbamate, cyclizing intramolecularly in acetic acid in the presence of KOAc under refluxing for 2 h, and hydrolyzing and decarboxylating with 36% HCl and mercaptoacetic acid under refluxing for 8 h, etc.