The invention relates to a method used for preparing a viscous oil viscosity reducer by taking naphthenic acid as a raw material. Naphthenic acid is widely available. The method comprise following steps: naphthenic acid and an equal amount or an excessive amount of N,N-dimethyl-1,3-propylene diamine are subjected to amidation so as to obtain a naphthenic acid amide, wherein the terminal group of the naphthenic acid amide contains tertiary amine; and the naphthenic acid amide is reacted with 1,4-butanedioldiglycidyl ether at a molar ratio of 2:1 so as to obtain a gemini-type amphiphilic molecule containing quaternary ammonium ions. The viscous oil viscosity reducer possesses ideal emulsifying capacity on viscous oil. The method is simple; cost is low; and a novel approach for viscous oil viscosity reducer preparation is provided.