The invention provides a fixture for machining a turned groove of a heated gear ring of a high precision synchronizer. The fixture is mainly composed of a transition connecting disc, a fixture body, a positioning disc, an expansion sleeve, a connecting column, three rotating pull rods, a pressing block, a pressing disc, a spring, a pushing disc and a guide screw. According to the fixture, a workpiece can be clamped on the expansion sleeve, the pressing block can rotate anticlockwise and compress the workpiece through matching of the rotating pull block with the guide screw, workpiece centering and end face compressing are achieved rapidly, the machining efficiency and the machining quality are improved greatly, and operation is convenient and rapid; machining of the turned groove of the thin-wall workpiece, especially machining of the heated workpiece is facilitated, and the fixture has the outstanding advantage in quality guaranteeing; and the fixture is not only suitable for the thin-wall workpiece, but also suitable for a non-thin-wall workpiece.