The absolute positioning accuracy of industrial robots is not in the high level to meet machining precision requirements. To improve the practical application requirements of industrial robots in the field of aerospace manufacturing, an online error compensation method based on the external Guided Motion (GEM) function of the milling robot is proposed in this paper. Using external measuring equipment, an external closed-loop control system of the robot is established, and the error compensation scheme is designed. The precision compensation of the robot’s end position is mainly studied. For the linear and circular motion of the robot, a continuous trajectory piecework compensation method is studied by using a PID control algorithm, which can measure the end position error online. The position errors of the robot are compensated simultaneously in the X, Y, and Z directions. Experiments show that the online error compensation method can improve the absolute accuracy of the robot end.