A numerical simulation of the dynamic control process in an AO system is carried out to complete a comprehensive numerical simulation of an AO system. In comparison to the theoretical analyses of automatic control, the numerical simulation of dynamic control process in an AO system is advantageous. The frequency response characteristics are closely related to the dynamic control performance of an AO system. A numerical simulation of the frequency response characteristics in an AO system is presented as well. The numerical simulation results agree with the experimental data excellently. The simulation calculation results of frequency response by using composite input of multiple frequencies are comparable to the single-frequency simulation results quite well and usable to determine the practical parameters of bandwidth and margins. The computing time of multi-frequency simulation calculation is much shorter than that of the single-frequency simulation calculation. The performances of an AO system can be effectively investigated by means of a combination of the numerical simulation of frequency response characteristics with that of long-exposure Strehl ratio.