The invention provides a preparation method of a fluorescent whitening agent. The preparation method comprises the following steps: mixing 4-dimethylaminobenzoylchloride, p-
chlorophenol ortho-aminophenol, doped SO42- / ZrO2-Fe2O3-SiO2 type
mixed crystal solid superacid catalyst and
ethylene glycol monobutyl
ether, heating to 160-180 DEG C, performing
reflux reaction for 2-3 hours, standing, separating the catalyst, adding 5-10 parts of
sodium hydroxide, rising to 100-120 DEG C, stirring for 1-2 hours, standing, separating and removing the lower solution, distilling off a
solvent from an upper
reaction product through a decompression method, enabling the reactant to a jelly, adding water below 10 DEG C, washing and filtering, and obtaining the fluorescent whitening agent which is 2-(p-dimethylamino phenyl)-4-
chlorine-
benzoxazole. The preparation method of the fluorescent whitening agent is simple in process; the use of the strong acid is avoided, so that the
corrosion to the equipment is avoided; the use of a large amount of water is avoided; the prepared fluorescent whitening agent is good in compatibility with a surface
sizing agent in
papermaking and good in whitening effect.