The invention discloses a mechanical arm for electrolytic machining of corrosion defects in a pipe, and the mechanical arm is used for machining the interior of the pipe. The mechanical arm comprises an upper base 14, an upper mechanical arm body 16, a lower mechanical arm body 2, a wrist 5, a mechanical claw 7 and a cathode corrosion mold 9. An upper rotary plate 3 is fixed to the tail end of the lower mechanical arm body 2, a lower rotary plate 4 is fixed to the top end of the wrist 5, and the lower rotary plate 4 and the upper rotary plate 3 are connected and can rotate around the central axis where the lower mechanical arm body 2 is located. An electrolyte channel is formed between the upper surface and the lower surface of the cathode corrosion mold 9, and an electrolyte inlet guide tube 10 communicating with the electrolyte channel is fixed to the upper surface of the cathode corrosion mold 9. The electrolytic machining on the corrosion defects in the pipe is carried out by continuously injecting electrolytes through the electrolyte channel with the cathode corrosion mold 9 as a cathode and a machining pipeline as an anode.