To minimize the error between actual processing trajectory and theoretical trajectory in computer numerical control (CNC) machine tool, an error optimization method with the redundant feature of six-axis linkage CNC machine tool was proposed. Firstly, the kinematic model of a type six-axis linkage CNC machine tool which had three linkage axes and three rotation linkage axes was established. Then, the feature of redundant linkage of machine tool was researched, and the redundant rotation linkage axis was defined. With the trajectory model of contact point between tool and workpiece, the trajectory errors models of tool center point and axis vector were established. The errors models showed that the values of them depended on the motion of rotation linkage axes. The motion of redundant rotation linkage axis was applied to adjust the motions of all linkage axes. The values of the two errors became the minimum when the sum of the motion of rotation linkage axes became the minimum. And, it made the values of the processing trajectory error became the minimum at the same time. To demonstrate the validity of the optimization method, two polishing experiments were made in a six-axis linkage CNC machine tool. An experiment was made in the five-axis linkage mode that the redundant rotation linkage axis was motionless, and another experiment was made in the six-axis linkage mode. Because the lower machining trajectory error made the lower surface roughness and the higher of its consistency, compared with the five-axis linkage mode, the processing result in six-axis linkage mode was more excellent. The validity of the method was verified by the results of experiments.