The basic working principle and mathematical model for a fast-steering mirror driven by PZT, which is a type of mirror widely used in optical-axis control systems, are analyzed, and the identification process of genetic algorithms in the model is described. A system for measuring the frequency response of a large-aperture fast-steering mirror driven by PZT is set up to realize parameter identification for the transfer function. The frequency response of the identified transfer function and the actual measured data are compared, and the identification accuracy of multiple resonance points in the system is analyzed. The experimental results show that at each resonance point, the frequency response of the transfer function obtained by the genetic algorithm is consistent with the measured data. The results also show that the transfer function obtained by this algorithm can analyze the dynamic characteristics of optical axis control system more accurately, and we can design advanced control algorithm, thus improving the performance of the optical-axis control system.