In the milling process, significant machining errors may occur due to the low stiffness of thin-walled parts. To reduce the cutting force-induced deformation in milling thin-walled parts such as blades, a cutter orientation optimization algorithm based on stiffness matching is proposed. The concept of maximum stiffness direction is put forward according to the phenomenon that different deformations are produced when applying forces with the same magnitude but in different directions. The stiffness of the in-process workpiece is obtained using the stiffness updating method. The cutter orientation optimization algorithm is presented to match the force with the maximum stiffness direction. The best cutter orientation is obtained by adopting the quantum particle swarms optimization algorithm at the key cutter location points, and then the cutter orientations of all cutter location points are obtained by the quaternion interpolation algorithm. The proposed deformation control method is verified on thin-walled blade milling experiments, and the experimental results show that the machining deformation of the blade with the optimized cutter orientation is reduced by about 36.51%, indicating that the proposed method can effectively reduce the machining deformation of the thin-walled parts.