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Spin-forming of cup-shaped thin-walled trapezoidal inner-gear is a new technology of the near-net forming in gear manufacturing field. Experiments and FEM simulations shows that the quality of workpiece is greatly influenced by the forming process method. The defects, such as the un-uniform distribution of tooth height along the axial and tangential directions and the wave-shaped opening-end of workpiece etc., occur easily. The mechanism for defects during spinning is analyzed, which is mainly caused by the partial thickening (in the area of gear-tooth) and thinning (in the area of tooth-groove). Improved forming methods, such as stagger spin-forming, positive-reverse rotation of main spindle and adopting a restraint ring at the opening-end of blank, are put forward. The forming methods are simulated by MSC. Marc and also investigated by experiments. The results show that the improved process methods overcome the forming defects effectively.