A swing frame unit of a swing includes a supporting frame, a seat frame disposed on the supporting frame, a backrest frame pivoted to the supporting frame and pivotable between a horizontal position and an inclined position, and an extension frame mounted movably on the supporting frame and movable in a transverse direction relative to the supporting frame from a retracted position to an extended position, in which the extension frame extends outwardly of the supporting frame in the transverse direction so as to increase an overall length of the backrest frame, the seat frame, and the extension frame in the transverse direction.