A two-level atom coupling with a single-mode thermal field through two-photon processes is considered. The reduced entropy change of the atom and field was studied by using quantum reduced entropy, and entanglement between the atom and field was measured by using concurrence. By means of numerical computation method, the effect of the Stark shift and mean photon number in J-C model with mixed states on the reduced entropy change and entanglement was investigated. The results show that reduced entropy changes of the atom and field decrease under the influence of Stark shift. When the initial parameters are suitable, the complete exchange between the atomic and field reduced entropy occurs. In addition, the minimum values of the entanglement raise in the presence of Stark shift, which means that the atom is no longer disentangled from the field.