To set up a mechanical model of threepoint loading for the intellectual measuring equipment of cable tension, the cases for elastic support with two ends of hinged support and those of fixed support with consideration of the influence of the cable were systematically studied. The equations for the upper and lower limits of the cable tension T as well as those for the real measured values have been obtained. A more rational equation for online measuring the cable tension by “Threepoint Bending Method” has been derived through secondary measurement.