The invention relates to an industrial robot external-sensor-free external force detection method. The external force detection method includes the steps that equivalent force moments of the external force at joints of a robot are obtained according to a momentum deviation observer algorithm, the magnitude, the approximate direction and acted connecting rods of the external force can be obtained according to the equivalent force moments, joint driving force moment parameters and joint positions are only required to be read from joint motor drivers of the industrial robot, speed parameters are obtained in a differential mode through the position parameters, and the external force acting on the robot can be obtained through the three kinds of parameters with the momentum deviation observer algorithm. According to the industrial robot external-sensor-free external force detection method, industrial robot external force detection can be achieved with the external force detection algorithm on the premise that external sensors are not required, and an existing robot is not required to be greatly modified; acceleration parameters are not used in the momentum deviation observer algorithm, the external force detection reliability and the external force detection accuracy are greatly improved, and the industrial robot external-sensor-free external force detection method is of great significance on detecting robot impact and improving the safety of the robot, and is simple and high in reliability.