The invention relates to a 2-cyano pyrrolidine compound, and a preparation method and application thereof. The structural formula of the 2-cyano pyrrolidine compound is shown as formula I in the specification, wherein X is one of carbon, nitrogen, oxygen and sulphur; R is one of benzene ring, substituted benzene ring, alkyl and heterocyclic ring. The preparation method comprises the following steps: condensing glycine and different substituted aldehyde as the initial materials; hydrolyzing; condensing to obtain a 2-cyano pyrrolidine dipeptidyl peptidase-4 inhibitor for preparing an antidiabetic medicine.