The invention provides a mechanical arm for spraying the inner wall of large-diameter pipelines. The mechanical arm mainly comprises a mechanical arm support, a first mechanical arm joint, a second mechanical arm joint and a spray gun. By the adoption of the technical scheme, the mechanical arm support can be fixed to any moving platform through a supporting rod on the upper portion of the mechanical arm, a supporting rod on the right portion of the mechanical arm and a supporting rod on the left portion of the mechanical arm. The mechanical arm support, the first mechanical arm joint and the second mechanical arm joint of the mechanical arm work cooperatively, the spray gun can be moved and located in a four-freedom-degree mode, and the mechanical arm can adapt to the pipelines with the different diameters and irregular operation faces. The mechanical arm is simple in structure and flexible to operate, and blind spots do not exist in spraying operation.