Piezoelectric ceramic ( PZT ) driver is a key component in PZT micro-displacement systems, and the noise index determines the accuracy of the output displacement of the PTZ actuator. In fast steering mirror (FSM) systems, the response of noise in a mechanical system is bound to affect resolution of the system. This paper analyzes the PZT driver system from the perspective of coupled electro-mechanical. By establishing a mathematical model of the coupled electro-mechanical system, we determine the energy of the output noise of the drive system in the mechanical system and the transfer of the electrical noise energy on the electromechanical system actuator. It is then determined that the power consumption of the PTZ actuator is mainly composed of two parts: the energy used to drive the deformation of the system and the dissipation of heat due to structural damping. The noise index of the PTZ driver system is 192.3dB when the accuracy of the output displacement is 1 nm through the energy conversion. The method is universal to be applied to determine the system noise index in various mechatronic systems, as well as to more complex mechanical systems. © 2019 IEEE.