The utility model discloses a mechanical automatic arm. Moving base, a storage fixing base is fixedly mounted on one side of the movable base; a control box is arranged on the front surface wall of the movable base; a walking guide wheel is arranged on a walking guide rail fixedly mounted on the inner side wall of the movable base; the walking guide wheels are fixedly installed on the walking vehicle body. And the walking guide wheels are rotationally connected with a walking rotating motor arranged on the side wall of the walking vehicle body through rotating gears, a rotating disc is fixed to a main shaft of a rotating motor arranged in the walking vehicle body, and six rotating guide wheels are symmetrically installed on the side wall of the side, close to the walking vehicle body, of the rotating disc. Through the arrangement of the telescopic mechanical arm, the variable-amplitude hydraulic oil cylinder, the rotating disc, the walking vehicle body and the hydraulic machine, the problems that the mechanical arm cannot rotate freely, the working amplitude of the mechanical arm is small, use and operation are difficult, and the mechanical arm cannot move during fixed work are solved.