<p>The cable–beam transmissibility function (CBTF) describes the intrinsic dynamic characteristics of the stay cable under boundary disturbances. This function is defined as the dynamic response ratio of the cable response to the beam end anchorage response and has a frequency response property that is very similar to that of the frequency response function. Due to the limited number of measuring points on the main beam in an actual monitoring system, directly measuring the boundary response of each cable is impossible. Therefore, a response reconstruction method that interpolates the deflection curve equation of the main beam based on finite measuring points is proposed. Extensive research has been conducted on conventional methods for identifying time-varying cable force using acceleration measurements from stay cables, while the CBTF is based on the transfer relationship between cable–beam responses for parameter identification. Therefore, a time-varying frequency identification method using a window function and discrete-time Fourier transform is proposed. The stay cable, constituted by multiple parallel high-strength steel wires, exhibits dynamic variations in its second moment of area corresponding to real-time tension fluctuations under operational loading conditions. Thus, the bending stiffness in the identification process is compared with the theoretical bounds to improve the accuracy of the time-varying cable force identification. Numerical simulation results show that the accuracy of the proposed response reconstruction method and time-varying cable force identification method is satisfactory. The practicability of the proposed method is verified by monitoring data of a cable-stayed bridge.</p>