The cable?net structural system of FAST is a flexible tension cable net， consisting of a main cable net， several hydraulic actuators and controllers， which is the world’s largest span， the highest precision， and the first active shape?changing cable?net sys? tem. Its characteristic is that the cable net form can be adjusted according to requirements， but it also results in the cable boundary conditions constantly changing with the cable net form， which brings huge challenges to cable force identification. In order to accu? rately identify cable forces of the active shape?changing cable?net system， a method for identifying cable forces of variable elastic boundary supports is proposed. An equivalent single?degree?of?freedom model of the cable is established， and the mathematical ex? pressions of the cable frequencies between ideal hinge and elastic boundary support are derived. The first?order frequency is then corrected based on the first?order mode values at the mid?point and both ends of the cable. The cable force identification method of the active shape?changing cable?net system which is based on the string vibration theory is proposed. Numerical simulations are car? ried out to verify the accuracy of the proposed method， and parametric analyses are also conducted. The method is proved to be practicable and applicable through numerical simulations and field measurements to identify the cable force of the FAST cable net. The results show that the relative errors of cable force identification are within 1% in the numerical simulation and less than 5% in the field measurement. The method takes into account the complex boundary conditions of cables， avoids solving for unknown boundary constraint stiffnesses， and extends the engineering applicability of the traditional string vibration theory.