The invention provides a method for preparing an intermediate 1-(3-ethoxy-4-methoxyphenyl)-2-methylsulfonylethylamine (I), shown in a following figure, of Apremilast. The method comprises the following steps: (1) adding dimethyl sulfone in a suitable solvent, and performing a reaction with an organolithium compound at -40-10 DEG C for 0.5-2 hours so as to obtain a reaction liquid R1; (2) adding 3-ethoxy-4-methoxybenzaldehyde in a suitable solvent, and performing a reaction with a weak-nucleophilicity strong base at -40-10 DEG C for 0.5-2 hours so as to obtain a reaction liquid R2; (3) adding the R2 into R1 dropwise, and performing a reaction at -60-0 DEG C for 0.5-2 hours so as to obtain a reaction liquid R3; (4) adding boron trifluoride diethyl etherate to the R3, and performing a reaction at -80-0 DEG C; (5) quenching the reaction, performing filtration, performing pulping and washing on the obtained filter cake by using dichloromethane, performing extraction on the aqueous layer with the dichloromethane, combining the organic layer, and performing concentration; (6) adding dichloromethane to the residues obtained after concentration, performing washing with acid water, and performing liquid separation; and (7) adjusting the pH value of the water layer to 10-14, performing extraction with dichloromethane, and concentrating the organic layer so as to obtain the 1-(3-ethoxy-4-methoxyphenyl)-2-methylsulfonylethylamine crude product. The method is based on a synthetic method firstly reported by an original research company, the feeding mode and feeding ratio are changed, control on reaction conditions is achieved, and the method has high operability, and is conducive to large-scale industrial production.