The exact dynamics of an open quantum system consisting of one qubit driven by a classical driving field is investigated.Our attention is focused on the influences of single-and two-photon excitations on the dynamics of quantum coherence and quantum entanglement.It is shown that the atomic coherence can be improved or even maintained by the classical driving field,the non-Markovian effect,and the atom-reservoir detuning.The interconversion between the atomic coherence and the atom-reservoir entanglement exists and can be controlled by the appropriate conditions.The conservation of coherence for different partitions is explored,and the dynamics of a system with two-photon excitations is different from the case of single-photon excitation.