Spin-forming of trapezoidal inner gear is a new technology of the near-net forming in gear manufacturing field. Processing parameters, such as the initial thickness of blank and radial reductions of each roller, greatly influence the forming quality of spun part. The forming quality of trapezoidal inner gear involves the filling status of gear tooth and dimensional accuracy of root circle. An approach for processing parameter optimization of the forming quality of the trapezoidal inner gear stagger spinning is proposed. The proposed approach integrates the orthogonal experiment design, gray relational analysis, and analysis of variance (ANOVA). The experiment result indicates that the proposed approach is effective in determining the optimized processing parameters for the stagger spinning of trapezoidal inner gear.