The evolution of the linear entropies of field, atom and field atom coupling system in the system of a two level atom inside a phase damping cavity interacting with a coherent field under large detuning are investigated. The effects of atomic initial state and the mean photon number of the field on the above linear entropies are discussed. The results show that the linear entropies of both the field and the atom are strongly dependent on atomic initial state, but the evolution of the linear entropy of the field atom coupling system is independent of the atomic initial preparation. With the increasing of the mean photon number, the linear entropies of field, atom and field atom increase.