The invention provides a method for preparing 3,4,5-trimethoxy-3'-hydroxy-4'-alkoxy diphenylethane. The method comprises the steps of firstly enabling 3-benzyloxy-4-hydroxybenzaldehyde and alkyl bromide to be subjected to alkoxylation reaction, so as to obtain a compound (2) shown in a drawing, then, enabling 3,4,5-trimethoxybenzyl triphenyl phosphine bromate and the compound (2) to be subjected to Wittig reaction, so as to prepare a compound (4) shown in a drawing, and finally, carrying out reduction on the compound (4) by palladium on carbon / hydrogen gas at normal pressure, thereby obtaining the target compound 3,4,5-trimethoxy-3'-hydroxy-4'-alkoxy diphenylethane. According to the method, the synthesis method is simple and reasonable, the operation is simple and convenient, the yield is high, the cost is low, the use of high-toxicity solvents, such as benzene, is avoided, the method is environmental-friendly, and the industrial production is facilitated.