A preparation method of N-alkyl-(3, 4-methylenedioxy) aniline relates to the method of the N-alkyl-(3, 4-methylenedioxy) aniline. 3, 4-(methylenedioxy) nitrobenzene in a pressure kettle is taken as the starting raw material, C1 to C4 lower aliphatic alcohols are taken as reaction solvents and alkylation reagents, a reductive alkylation catalyst is added, the pressure kettle is sealed, hydrogen gas is introduced for replacement, the temperature is increased, the hydrogen is introduced, the hydrogenation reaction is finished, the temperature is decreased, the pressure is relieved, then the N-alkylation reaction, the nitrogen introducing for replacement and the temperature increase reaction are continuously carried out for preparing the N-alkyl-(3, 4-methylenedioxy) aniline. In the preparation method, the hydrogenation reduction and the N-alkylation reaction are carried out in a same reaction kettle, the reduction products do not need to be separated, aldehyde and ketone alkylation reagents do not need to be added, and the N-alkylation reaction is directly carried out. The preparation method has small environmental pollution, good N-alkylation selectivity and high conversion rate, the raw material is easy to be recovered, the catalyst can be applied, the production cost is low, and the production process can be simplified.