The invention relates to a high-grade fluorescent whitening agent widely used in the washing industry, and concretely relates to an environmentally-friendly preparation method of a fluorescent whitening agent. The environmentally-friendly preparation method of the fluorescent whitening agent comprises the following steps: (1) taking 4,4'-bis(chloromethyl)-1,1'-biphenyl and triethyl phosphite, performing heating for a refluxing reaction in the presence of a protection gas N2, distilling out unreacted triethyl phosphite and byproducts under a reduced pressure, and carrying out condensation and crushing to obtain an esterified product; (2) taking the esterified product and sodium 2-formylbenzenesulfonate, and carrying out a reaction under the action of a strong polar aprotic organic solvent and an organic strong alkali to obtain a condensate, solvent and salt mixture; (3) distilling the condensate, solvent and salt mixture, and recovering the solvent to obtain a high-solid condensation mixture; (4) adding water and sodium chloride to the high-solid condensation mixture, cooling the obtained mixture, carrying out press filtration to obtain a salting-out filter cake, and washing and refining the salting-out filter cake; and (5) drying the refined filter cake by a flash evaporation dryer.