The invention discloses a self-centering fixture which comprises a mandrel and an expansion sleeve, wherein the mandrel is mounted on a machine tool through a flange; the expansion sleeve is sleeved on the outer circumference of the mandrel and can move axially; multiple expansion joints are arranged on the outer wall of the expansion sleeve; the outer wall of the mandrel and the inner wall of the expansion sleeve are matched conical surfaces; the mandrel is provided with an adjusting nut for driving the expansion sleeve to move along the mandrel; a pull rod and a stop ring are arranged between the mandrel and the expansion sleeve to pull the expanded sleeve to a shrinkage end; and through the expansion and shrinkage of the expansion joints, the outer diameter is controlled to increase/reduce so as to position/dismantle a workpiece. The fixture has a simple structure, is convenient to use and can realize self centering; and moreover, the processing accuracy of the workpiece is improved, the concentricity and verticality between the processed surface and an inner hole of the workpiece can be ensured, and the fixture deserves popularization and application in the fields realizing positioning and clamping by use of the inner hole of the workpiece.