In this paper, the interactions between a two-level atom and a coherent field with a time-varying frequency have been studied. The two typical cases, in which the frequency of the field varies with time in the form of sine or impulse, have been considered. It is found that the collapse and revival phenomena of the atomic population inversion will disappear as the frequency changing with time in sine form; and the frequency varying in impulse form can lead to some new collapses and new revivals in the evolution of the atomic population inversion.