Inner gear spinning technology with three rollers was analyzed. According to the material flowing characteristics during spin-forming, two forming quality evaluation factors were put forward and then filling status of gear tooth under different reductions was analyzed. The results indicated that there was an optimum reduction for blank with certain thickness during inner gear spinning. When the reduction was below, the filling status in gear tooth was unsatisfied due to lack of filling material; when the radial reduction was above, the increasing forming force induced by higher reduction would likely decrease the service life of forming tool, and excess relative reduction would result in the status of underfills in the gear tooth. 