The invention relates to a locking and releasing device for a large connecting piece and belongs to the technical field of mechanical engineering. The locking and releasing device comprises a gear nut, a constant power mechanism, an automatic centering mechanism, a lifting mechanism and a free following mechanism, wherein the constant power mechanism is used for supplying constant power to the gear nut, and the gear nut is enabled to keep consistent torque in both locking and releasing processes; the automatic centering mechanism is used for automatically adjusting the centering amount in a rotating process, the centering position is guaranteed, and a part is prevented from grinding and damage; the free following mechanism is used for guiding and supporting the part in the locking and releasing processes; the lifting mechanism is used for adjusting the high-low positions; the constant power mechanism is mounted on the automatic centering mechanism which is mounted on the free following mechanism, and the free following mechanism is mounted on the lifting mechanism. According to device, locking and releasing of the large connecting piece are simple, rapid, stable and accurate, and the problems of low assembly efficiency, inconsistent tightening torque and thread damage caused by centering and superpositioning of assembly are solved.