A system for joining buttress elements to the main shaft of a wind turbine tower, wherein: - the main shaft is made up of rings; - the main shaft comprises an upper joining ring and a lower joining ring; - each buttress element comprises an end support piece with a flat end; - the upper joining ring comprises a first perimeter surface and a first support surface that forms an angle (α) with the first perimeter surface; and - the lower joining ring comprises a second perimeter surface and a second support surface parallel to the first support surface of the upper joining ring such that it forms an angle (α) with the second perimeter surface of identical value to the angle (α) of the first support surface with the first perimeter surface; such that the flat end of each buttress element is fastened between the first support surface of the upper joining ring and the second support surface of the lower joining ring, and wherein the angle (α) has a value comprised between 60° and 120°; - the second support surface of the lower joining ring comprising a first through hole and the flat end of the buttress element comprises a first through hole wherein the first through holes (11B, 11C) house a first bolt that joins the flat end of the buttress element to the lower joining ring; - the first support surface of the upper ring comprising a second through hole, the flat end of the buttress element comprises a second through hole and the lower joining ring comprises a second through hole, wherein the second through holes (13A, 13B, 13C) house a second bolt that joins the upper joining ring to the lower joining ring with the flat end of the buttress element between the two joining rings (3A, 3B); characterized in that: - the first through holes (11B, 11C) being located inside the perimeter of both the first perimeter surface and the second perimeter surface; and - the second through holes (13A, 13B, 13C) being located inside the perimeter of both the first perimeter surface and the second perimeter surface. - the main shaft is made up of rings; - the main shaft comprises an upper joining ring and a lower joining ring; - each buttress element comprises an end support piece with a flat end; - the upper joining ring comprises a first perimeter surface and a first support surface that forms an angle (α) with the first perimeter surface; and - the lower joining ring comprises a second perimeter surface and a second support surface parallel to the first support surface of the upper joining ring such that it forms an angle (α) with the second perimeter surface of identical value to the angle (α) of the first support surface with the first perimeter surface; such that the flat end of each buttress element is fastened between the first support surface of the upper joining ring and the second support surface of the lower joining ring, and wherein the angle (α) has a value comprised between 60° and 120°; - the second support surface of the lower joining ring comprising a first through hole and the flat end of the buttress element comprises a first through hole wherein the first through holes (11B, 11C) house a first bolt that joins the flat end of the buttress element to the lower joining ring; - the first support surface of the upper ring comprising a second through hole, the flat end of the buttress element comprises a second through hole and the lower joining ring comprises a second through hole, wherein the second through holes (13A, 13B, 13C) house a second bolt that joins the upper joining ring to the lower joining ring with the flat end of the buttress element between the two joining rings (3A, 3B); - the first through holes (11B, 11C) being located inside the perimeter of both the first perimeter surface and the second perimeter surface; and - the second through holes (13A, 13B, 13C) being located inside the perimeter of both the first perimeter surface and the second perimeter surface.