The fast steering mirror is now being used in industries beyond precision processing, such as space and defense. The piezoelectric fast steering mirror (PFSM), which utilizes a piezoelectric actuator, is particularly suitable for these industries as they often require devices like electro-optic devices to withstand external vibrations and impacts. While the PFSM has inherent high stiffness, its complex structure makes it difficult to control. To address this, an accurate dynamic model is necessary. In this paper, we derived a dynamic model for the PFSM using a two-inertial system model that takes into account its structural characteristics. This dynamic model consists of both a mechanical system model and an electrical system model. We measured the frequency response function from electrical input to mechanical output and compared it with the derived frequency response model to verify its accuracy. The derived model can be used not only for control design, but also for instrument design and interpretation.