The invention discloses a rotary joint positioning and locking mechanism, a working method and a line robot. The rotary joint positioning and locking mechanism comprises a driving motor set, a lead screw guide rail set and a guide clamp, wherein the driving motor set, the lead screw guide rail set and the guide clamp are connected in sequence, and the lead screw guide rail set drives the guide clamp to run back and forth in the direction of a guide rail; first sliding devices which can move relative to the inner side of the opening of the guide clamp are symmetrically arranged on a rotating shaft, the guide clamp gets contact with the first sliding devices in the operation process and moves relative to the first sliding devices, and the rotating shaft is driven to rotate directionally; and the rotating shaft and a rotating joint fixing end are coaxially arranged, the rotating joint fixing end is provided with a first limiting device, and the rotating shaft rotates in a range limited by the first limiting device. The rotary joint positioning and locking mechanism has the beneficial effects that rotary joints in a certain rotating angle range can be positioned and locked, and the positioning and locking of the rotary joints of the mechanical arms of the robot under the special requirement is achieved.