Abstract: With a sub-iterative technique for fluid dynamic equations and rigid-body dynamic equations,a coupling numerical method is proposed to explore the unsteady movement characteristics of aero-crafts and the effect of coupling time accuracy.It is shown that the motion finally becomes a self-maintained stable limit cycle oscillation.The amplitude with an increasing incidence angle shows a jump,which is quite close to the experimental result.The coupled numerical result with the second-order time accuracy depends hardly on the physical time step for this multi-system coupling.On the contrary,the uncoupled numerical result with one step time lag depends obviously on the physical time step selected.A longer time step results in unreal numerical results.