The invention provides a contact plug structure. The contact plug structure comprises a substrate and a dielectric layer, and a first contact hole located in the dielectric layer and penetrating into the substrate, the first contact hole has a first through hole portion located in the dielectric layer and a first groove located in the substrate, and the first through hole portion is communicated with the first groove, the maximum width of the first groove is larger than that of the first through hole portion in the direction parallel to the substrate surface. A barrier layer at least partially covering the sidewall of the first groove. A conductive pad layer is located on the bottom surface of the first groove. The conductive core layer is arranged on the conductive pad layer, and the barrier layer wraps the conductive pad layer and the conductive core layer.