The invention discloses a preparation method of a midbody for preparing anti-multiple myeloma pomalidomide. The preparation method comprises the following steps: dissolving a compound as shown in a formula III in a solvent, reducing by using hydrogen in the existence of a palladium carbon catalyst, and then collecting a compound as shown in a formula IV from a reaction product, the compound as shown in the formula IV is the midbody, namely 3-amion-[(2,6-dioxo-3-piperidyl) carbamoyl group] benzoic acid and esters thereof for preparing the pomalidomide. The preparation method has the advantages that reaction steps are few, conditions are mild, the aftertreatment is simple, the yield is high, the preparation method is suitable for large-scale industrial production, and the reaction formula is as shown in descriptions.