Abstract: The development and utilization of marine resources and space are two major themes of human development of the ocean in the 21st century. As an important force-supporting and anchoring component of marine equipment and deep-sea structures, the damage and destruction of anchor cable will directly affect the safety and durability of the whole structure, so it is necessary to monitor and evaluate the service state of anchor cable in real time. Cable force is an important physical quantity reflecting the static and dynamic characteristics of anchor cable. It is of great significance to master the real-time change of cable force for the health monitoring and state assessment of anchor cable. The existing researches usually use modified cable force formula or intelligent optimization algorithm to identify cable force on the basis of tension string or simply supported beam model, but fail to fully consider the geometric nonlinearity caused by sag. In order to give a more explicit physical meaning and more accurate identification formula of anchor cable force in theory, aiming at the geometric nonlinearity and damping nonlinearity of underwater anchor cable, firstly, the perturbation solution of free vibration frequency and response of anchor cable is derived by using equivalent linearization technique, and the analytical expression of frequency considering sag of anchor cable is given. On this basis, the cable force identification scheme based on vibration method is given. Numerical examples show that the recognition results of this method are consistent with the real values, thus verifying the accuracy of this method. The relevant theories and conclusions can provide theoretical basis for dynamic analysis and health monitoring of such engineering structures.