The invention discloses an adjustable stability improving device of a hypersonic aircraft. The device comprises first tail skirts, second tail skirts, a first shaft sleeve, a second shaft sleeve and acenter shaft, wherein the first tail skirts and the second tail skirts are formed by a plurality of partially overlapped scales in a surrounding mode, and the scales of the first tail skirts are movably connected with the tail end of the aircraft and surround the aircraft by a circle; the scales of all the first tail skirts are connected with the first shaft sleeve through a framework; the scalesof the second tail skirts are movably connected with the tail ends of the scales of the first tail skirts in the axial direction of the aircraft, and the scales of all the second tail skirts are connected with the second shaft sleeve through a framework; the center shaft is sequentially sleeved with the first shaft sleeve and the second shaft sleeve in the axial direction of the aircraft, and thetension angles of the first tail skirts and the second tail skirts are changed freely along the central axis, so that the flight stability of the hypersonic aircraft is adjusted.