The invention discloses a full-cross-section tunneling operation system and a segment assembly machine thereof. The segment assembly machine comprises a rotatable rotation frame, a segment grabbing part used for grabbing a segment and a large arm oil cylinder mounted on the rotation frame. The segment grabbing part is mounted on the large arm oil cylinder and move along with stretching out and drawing back of the large arm oil cylinder, a balance weight assembly is further included, and is mounted on the side, away from the segment grabbing part, of the rotation frame, and the balance weight assembly comprises a balance weight which can move in the direction away from or close to the rotation frame. According to the segment assembly machine, the balance weight assembly is arranged on the rotation frame, moving of the balance weight is used, balance of load can be achieved, when the assembly machine rotates, only friction force of the assembly machine needs to be overcome, starting torque is provided, the property need of equipment can be effectively reduced, system pressure is reduced, energy is saved, and the application range is widened.