The invention relates to the field of chemical synthesis, and discloses a method for preparing lenalidomide, which comprises the following steps: condensing 3-aminopiperidyl-2,6-diketohydrochloride with 2-halomethyl-3-nitro-methyl benzoate under weakly alkaline conditions, cooling the reaction solution to room temperature, adding into water, stirring, filtering to obtain a filter cake, sequentially washing with water and alcohol reagents, and drying to obtain 3-(7-nitro-3-oxo-1H-isoindazolyl-2-yl)piperidyl-2,6-dione; and carrying out nitro-amino reduction reaction, filtering the reaction solution, concentrating, crystallizing at -20 DEG C to room temperature, filtering, washing the filter cake with alcohol reagents, drying to obtain a lenalidomide crude product, and recrystallizing to obtain the lenalidomide finished product. By adjusting the after-treatment technique, the invention enhances the total yield of lenalidomide, and ensures the high purity; and the whole method has the advantages of mild reaction conditions and no high temperature or high pressure, and can be used for preparing lenalidomide in a simple and efficient way. 2-halomethyl-3-nitro-methyl benzoate.