In order to control the gap between rotor and stator of permanent magnet low speed synchronous motor less than 0.05 mm,coaxiality of fine turning of ends rabbet of stator with winding which is the core component must be ensured. A set of double expansion clamping cylinder turning fixture integrated positioning and clamping functions was introduced. It overcame the shortcomings of the small taper mandrel lathe clamp, including demoulding difficulty,high scrap rate of defects. And the product quality was improved. Since the clamping force was from cylinder,the axial tension was changed into radial expansion force,thereby the production efficiency was greatly improved. Batch production proves the fixture has the characteristics of high production efficiency,easy operation,stable product quality.