Aiming at the nonlinear error problem caused by linear interpolation of rotation axis and tool axis vector deviation from the theoretical plane in the mirror milling of aviation thin-walled parts, a low-fluctuation interpolation tool axis vector method of rotation axis suitable for mirror milling was proposed to optimize the nonlinear error. Firstly, the motion chain model and nonlinear error model of mirror milling machine tool were established, and the positive correlation between nonlinear error and the motion of the rotation axis of the machine tool was obtained. Then, based on the kinematics of the machine tool, the optimization interval was determined by the nonlinear error constraint and the angular velocity constraint of the axis of rotation. Finally, for the pre-interpolated tool axis vector of the minimum surface of nonlinear error, the pre-interpolation vector corresponds to the discrete rotation axis angle point, and the interpolated tool axis vector satisfying the mirror milling was determined within the allowable range of nonlinear error. After simulation and experiment, the maximum nonlinearity error is reduced by 73.18%, the overall wall thickness error is reduced by 68.24%, and the processing path time is reduced by 4.39%, which effectively verifies the feasibility of the proposed method.