As an essential beam-splitting component in the beamline station of synchrotron radiation sources, the channel-cut crystal serves to improve reflectivity, collimation, and stability of the X-ray beam emitted from the synchrotron radiation source. However, a major limitation to the implementation of the channel-cut crystal is polishing its narrow channel. To resolve this problem, a novel chemical mechanical polishing motion pattern is proposed. In addition, a relative motion model between the workpiece and the polishing plate is established, the equation of the trajectory of the abrasives relative to the workpiece is derived, the effects of multiple factors such as motion cycle ratio between the workpiece and the polishing plate are analyzed, and eccentric radius on the relative motion trajectory is analyzed too. Furthermore, by identifying the effects of the polishing trajectory, the polishing solution concentration and polishing pressure on the polishing results were explored, and the mirror polishing of the inner surface of the narrow channel of the channel-cut crystal is realized successfully. The proposed motion patterns of CMP can be applied to polish narrow structures of hard and brittle material (e.g., silicon, glass).