By the method of coherent-state orthogonalization expansion, the evolution of atomic population inversion is investigated in the non-rotating-wave approximation. The numerical results indicate that, when field frequency and coupling do not change with time, the collapse-revival period of population inversion increases with the mean photon number of the field increasing, and the quick oscillation due to virtual photon process increases with couplings strengthening. When the frequency changes with time in the sine form, amplitude α and angle frequency β of the field both have a great influence on atomic population inversion. The sudden jumping of the field frequency can lead to some new collapses and rebibals in the evolution of atomic population inversion. Both of the field frequency forms can have an influence on quick oscillation due to virtual photon process.