A flat plate sample holder expansion structure for use in a vacuum, comprising an adapter, a flat plate sample holder tray and a cylindrical sample holder gripper, wherein: the adapter is configured to be detachably connected to the cylindrical sample holder gripper; the adapter is futher configured to be detachably attached to the flat plate sample holder tray; a receiving space for receiving a flat plate sample holder is formed between the adapter and the flat plate sample holder tray; and the flat plate sample holder is configured to be detachably and rigidly arranged in the receiving space; wherein the flat plate sample holder expansion structure for use in the vacuum further comprises a fastening element; characterized in that: the flat plate sample holder tray comprises an opening, and a stepped surface is formed on a side wall of the opening; the adapter is substantially an I-shaped structure, and a through hole is formed in a center of the I-shaped structure; the fastening element is configured to pass through the through hole, and to abut on the flat plate sample holder after the flat plate sample holder is installed on the stepped surface; a head portion of the I-shaped structure is disc-shaped and further comprises protruding rods arranged on an outer side wall of the disc-shaped head portion; and the cylindrical sample holder gripper is configured to be detachably connected to the adapter by the protruding rods; a rear portion of the I-shaped structure is cylindrical, and threaded holes are evenly arranged in a side wall of the cylindrical rear portion and configured to connect to the flat plate sample holder tray; and the rear portion comprises further locking ears extending outward in a radial direction on the side wall of the rear portion, and wherein the flat sample holder expansion structure comprises further a cylindrical sample holder locking base which is configured to be detachably connected to the adapter by the locking ears. the adapter is configured to be detachably connected to the cylindrical sample holder gripper; the adapter is futher configured to be detachably attached to the flat plate sample holder tray; a receiving space for receiving a flat plate sample holder is formed between the adapter and the flat plate sample holder tray; and the flat plate sample holder is configured to be detachably and rigidly arranged in the receiving space; wherein the flat plate sample holder expansion structure for use in the vacuum further comprises a fastening element; characterized in that: the flat plate sample holder tray comprises an opening, and a stepped surface is formed on a side wall of the opening; the adapter is substantially an I-shaped structure, and a through hole is formed in a center of the I-shaped structure; the fastening element is configured to pass through the through hole, and to abut on the flat plate sample holder after the flat plate sample holder is installed on the stepped surface; a head portion of the I-shaped structure is disc-shaped and further comprises protruding rods arranged on an outer side wall of the disc-shaped head portion; and the cylindrical sample holder gripper is configured to be detachably connected to the adapter by the protruding rods; a rear portion of the I-shaped structure is cylindrical, and threaded holes are evenly arranged in a side wall of the cylindrical rear portion and configured to connect to the flat plate sample holder tray; and the rear portion comprises further locking ears extending outward in a radial direction on the side wall of the rear portion, and wherein the flat sample holder expansion structure comprises further a cylindrical sample holder locking base which is configured to be detachably connected to the adapter by the locking ears.