Abstract: To explore the physical mechanism induced by a frequency-modulated light field, the phenomenon of the oscillation of population probability in a twolevel system under a frequency-modulated field is analyzed theoretically by means of the density matrix equation and Bloch vector model. The effect of the modulated amplitude on the process of the oscillation is also discussed. The results show that the oscillation of population probability and population transfer is modulated by the modulated amplitude. Increasing the modulated amplitude when the other parameters are fixed, the oscillation of population probability is weakened, and the efficiency of population transfer and the absorption are enhanced. The analysis in Bloch vector model shows that the population oscillation reflects the dynamic transformation process among the population transfer, absorption and polarization in the quantum system.