  Using the algebraic dynamical method, the dynamics of entanglement in an
atom-field bipartite system in a mixed state is investigated. The atomic
center-ofmass motion and the field-mode structure are also included in this
system. We find that the larger values of the detuning and the average photon
number, the smaller fluctuation of the entanglement, but the period for the
evolution of the entanglement doesn't increase accordingly; It is also found
that the fluctuation of the entanglement varies slightly with the atomic motion
and oscillates fast with the value of the fieldmode structure and the
transition photon number increasing. Moreover, a damping evolution of the
entanglement appears when considering detuning and the atomic motion
simultaneously.
