As compared with the conventional flow forming of tube-shaped parts, the spin-forming of cup-shaped thin-walled trapezoidal inner gear is more complicated. In order to improve the forming quality of inner gears and lay a foundation for the selection of forming parameters, numerical simulation and processing experiment were performed to analyze the material flow during the spinning and the corresponding forming mechanism. The results show that （ 1 ） the spin-forming of inner gears is an upset-extrusion process ; （2） the deformation distribution is periodical, nonuniform and dissymmetric ; （3） the forming of gear tooth is influenced by the rotation direction of the main spindle, and forming defects easily occur during the spinning.