The invention provides a clamping device and method used for guaranteeing equality of the extension lengths of the two ends of a through bolt. The clamping device and method mainly solve the problem that the extension lengths of the two ends of a through bolt are not equal when the through bolt is mounted manually. The clamping device comprises a magnetic base and a shell, wherein the magnetic base can be attracted to a flange, and the shell is perpendicularly arranged on the base; the shell is internally provided with a first cavity communicated with a through hole in the base, and the two opposite side faces of the shell are symmetrically provided with a first slot hole and a second slot hole; a ruler is arranged at one side of the first slot hole in the shell, and the zero graduation line of the ruler is flush with the bottom face of the base; the first cavity is internally provided with a first V-shaped block and a second V-shaped block which are provided with opposite openings; a threaded rod capable of rotating along the second slot hole is arranged at one side of the second slot hole; the part, on the outer side of the second slot hole, of the threaded hole is provided with a movable nut; the threaded rod is rotated, the first V-shaped block moves towards the second V-shaped block and clamps the through bolt, and the position of the threaded rod is fixed through the movable nut. The clamping device is simple in structure, convenient to use and capable of effectively guaranteeing the equality of the extension lengths of the two ends of the through bolt.