Provided is a fully hydraulically driven positioning mechanism with adjustable operation space and used for a friction stitch welding machine. The positioning mechanism comprises a welding support, a spindle supporting frame, an X-direction movement hydraulic cylinder, a spindle and a plurality of gap adjusting structures, wherein the spindle supporting frame, the X-direction movement hydraulic cylinder, the spindle and the gap adjusting structures are installed on the welding support; the spindle supporting frame comprises a liftable framework cross beam arranged inside the welding support, a spindle mounting frame connected to the interior of the framework cross beam in a sliding mode, and the two ends of the framework cross beam are each provided with a Z-direction driving cylinder; and each gap adjusting structure comprises a chamfered face formed at the outer corner of an end plate of the framework cross beam, the chamfered face extends in the whole height direction of the end plate and is provided with first wedge-shaped blocks which are arranged symmetrically up and down, and each first wedge-shaped block is provided with a second wedge-shaped block in a matched mode. The positioning mechanism can not only recover cracks, in complex shapes, of underwater tubular structure objects, but also greatly improve the positioning precision and response speed of the friction stitch welding machine, the friction stitch welding machine is not prone to being affected by the underwater environment, and a powerful guarantee is provided for underwater work.