The invention discloses a carbon fiber cluster sample preparation device and a carbon fiber cluster preparation method. The device includes a pre-pressing straightening assembly, a restraining ring assembly and a trimming assembly, wherein the pre-pressing straightening assembly is used for pressing and straightening carbon fibers forming a carbon fiber bundle to obtain a cylindrical carbon fiberbundle, the restraining ring assembly includes an upper restraining ring and a lower restraining ring, the upper restraining ring and the lower restraining ring are two semi-rings in the form of snaps, which are butted to form a full circle as a restraining ring for restraining the carbon fiber bundle, the trimming assembly includes a clamping device used for clamping the restraining ring and a cutter used for trimming the clamped carbon fiber bundle. The device is advantaged in that through pre-pressing straightening assembly, smoothness and collimation of a carbon fiber bundle state can be more in line with experimental requirements; through the restraining ring, the carbon fiber bundle sample can be made to acquire accurate volume density and profile dimensions, and the sample preparation device can greatly reduce interference of the sample preparation process of the sample on the experimental result.