The invention relates to the technical field of industrial robots, and discloses a robot mechanical arm. The robot mechanical arm comprises a fixed base, a servo motor and a precise harmonic reducer.The mechanical arm can rotate in the YZ plane, a movable shoulder and a movable elbow are conveyed to the side edge of a large part, then the position and posture of a welding gun are adjusted throughthe movable shoulder, the movable elbow, a sliding rod and an executive part connecting rod so as to reach the position needing welding, the freedom degree of the welding robot is increased, the welding range is widened, and the limitation of the welding robot by the size of a welding part is reduced; meanwhile, due to the arrangement of the sliding rod, when the welding gun carries out linear welding on the welded part, direct control can be carried out through linear advancing, cooperative use of two or more mechanical arms is reduced, the complex degree of the moving track of the mechanical arms is reduced, and the calculated amount and workload of workers are reduced; and the complexity of program control is reduced, the operation speed of the mechanical arm is increased, and the welding efficiency and quality are improved.