An automatic calibration method for a robot system comprises providing a ball-rod member including a connection rod and a sphere connected to a first end of the connection rod, fixing an opposite second end of the connection rod to an end execution tool mounted on a flange of a robot, and controlling the robot to move a center of the sphere to a same target point in a plurality of different poses under the guidance of a vision sensor. A 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 same target point. A transformation matrix of a center of the end execution tool with respect to the center of the flange is calculated according to a formula.