<p>To address the issue of poor machining accuracy caused by the low stiffness and high deformation susceptibility of thin-walled workpieces, a variable-stiffness milling system was developed based on a magnetorheological (MR) auxiliary support device through structural design and magnetic field simulation. A dynamic milling model of thin-walled workpieces with distributed mass was established using the modal superposition method, incorporating the characteristics of the MR support system. The stability of the system was analyzed through numerical integration, and stability lobe diagrams were generated to investigate the influence of excitation current and the spatial position of the thin-walled workpiece on milling stability. Theoretical analysis results indicate that the adjustment of the system’s dynamic characteristics using the proposed MR auxiliary support device can effectively enhance the stability of the thin-walled milling process and reduce surface roughness. However, the limiting axial depth of cut at a given spindle speed varies with changes in excitation current and machining position. To validate the theoretical predictions, a milling test platform was constructed, and cutting signals along with surface roughness data were collected. The experimental results demonstrated good agreement with the theoretical predictions, confirming the effectiveness of the proposed approach.</p>