A contact window structure, a metal plug and a forming method thereof, a method of forming the contact window structure and a semiconductor structure are provided. In the method of forming the contact window, an annular pad is formed on a surface of a target layer. A central via, from which partial surface of the target layer is exposed, is formed in the middle part of the annular pad. A dielectric layer covering a substrate, the target layer and the annular pad is formed. The dielectric layer is etched to form an etch hole connected to the central via in the dielectric layer. The annular pad is removed along the etch hole and the central via to enlarge a size of the central via, so as to form the contact window structure by the etch hole and the central via with the enlarged size.