An automatic calibration method of a robot system, comprising steps of: S100: providing a ball-rod member (41, 42) comprising a connection rod and a sphere connected to one end of the connection rod; S200: fixing the other end of the connection rod to a terminate execution tool mounted on a flange of a robot; S300: providing a first distance sensor, a second distance sensor and a third distance sensor around a known target point, so as to sense three actual distances (L1', L2', L3') from the three distance sensors (11, 12, 13) to a surface of the sphere, respectively, the first distance sensor, the second distance sensor and the third distance sensor being distanced from the surface of the sphere by a first predetermined distance, a second predetermined distance and a third predetermined distance, respectively, when the center of the sphere is accurately moved to the target point; S400: controlling the robot to move the center of the sphere to the target point in various different poses (pose#1, pose#2, pose#3, pose#4) based on a first distance error between a first actual distance (L1') sensed by the first distance sensor and the first predetermined distance, a second distance error between a second actual distance (L2') sensed by the second distance sensor and the second predetermined distance, and a third distance error between a third actual distance (L3') sensed by the third distance sensor and the third predetermined distance, by performing a closed-loop feedback control on the robot until the first distance error, the second distance error and the third distance error all become zero, and calculating a transformation matrix Ts of the center of the sphere with respect to the center (ToolO) of the flange based on pose data of the robot at the target point in the various different poses; and S500: calculating a transformation matrix Tt of the center (Tool) of the terminate execution tool with respect to the center (ToolO) of the flange according to a following formula: Tt = Ts ∗ Tc wherein Tc is a transformation matrix of the center (Tool) of the terminate execution tool with respect to the center of the sphere, and Tc is known and constant. S100: providing a ball-rod member (41, 42) comprising a connection rod and a sphere connected to one end of the connection rod; S200: fixing the other end of the connection rod to a terminate execution tool mounted on a flange of a robot; S300: providing a first distance sensor, a second distance sensor and a third distance sensor around a known target point, so as to sense three actual distances (L1', L2', L3') from the three distance sensors (11, 12, 13) to a surface of the sphere, respectively, the first distance sensor, the second distance sensor and the third distance sensor being distanced from the surface of the sphere by a first predetermined distance, a second predetermined distance and a third predetermined distance, respectively, when the center of the sphere is accurately moved to the target point; S400: controlling the robot to move the center of the sphere to the target point in various different poses (pose#1, pose#2, pose#3, pose#4) based on a first distance error between a first actual distance (L1') sensed by the first distance sensor and the first predetermined distance, a second distance error between a second actual distance (L2') sensed by the second distance sensor and the second predetermined distance, and a third distance error between a third actual distance (L3') sensed by the third distance sensor and the third predetermined distance, by performing a closed-loop feedback control on the robot until the first distance error, the second distance error and the third distance error all become zero, and calculating a transformation matrix Ts of the center of the sphere with respect to the center (ToolO) of the flange based on pose data of the robot at the target point in the various different poses; and S500: calculating a transformation matrix Tt of the center (Tool) of the terminate execution tool with respect to the center (ToolO) of the flange according to a following formula: Tt = Ts ∗ Tc wherein Tc is a transformation matrix of the center (Tool) of the terminate execution tool with respect to the center of the sphere, and Tc is known and constant.