To meet the design requirements of a fast steering mirror (FSM) for an infrared search system, the electromechanical co-simulation technology of FSM based on the cross-reed transmission structure and voice coil actuator was studied. An electromechanical parametric model of the FSM was established, the transmission stiffness model of the flexible structure was constructed by the finite element analysis, the working model of the voice coil motor was constructed, and the key design parameters were compared and iterated to determine the optimal design parameters. MATLAB/Simulink was used as a co-simulation platform to establish the FSM dynamics simulation and electromagnetic drive simulation interfaces, combined with the classical control model to realize the co-simulation of the FSM and obtain the simulation results of the system dynamic response. Finally, the flyback compensation residual and phase lag under a 50-Hz imaging period were verified experimentally. The results show that the measured flyback compensation residual is 0.0365 mrad and the phase lag is 2.6 ms, which are higher than the simulation analysis results but meet the requirements of engineering applications. The open-loop frequency response curve of the system was compared, and the mid-low frequency amplitude response error did not exceed 10%. The simulation and experimental results show that this co-simulation technology has important theoretical significance for the design and optimization of FSMs.