An automatic calibration method of a robot system comprises providing a ball-rod member including a connection rod and a sphere connected to the connection rod, fixing the connection rod to an end execution tool mounted on a flange of a robot, providing distance sensors around a target point, and sensing an actual distance from each of the distance sensors to the sphere. The robot is controlled to move a center of the sphere to the target point in different poses based on the actual distances sensed by the distance sensors. A first transformation matrix of the center of the sphere with respect to a center of the flange is calculated based on pose data of the robot at the target point. A second transformation matrix of a center of the end execution tool with respect to the center of the flange is calculated based on the first transformation matrix.