Vigilant attention plays an important role in some industries and everyday life. However, the relationship between the variation of vigilant attention and phase synchronization is still unknown. This study utilized a revised version of the psychomotor vigilance test (PVT) to elicit vigilance decrement while collecting electroencephalogram (EEG) and using inter-site phase clustering (ISPC) to analyze phase synchronization. The theta tACS modulates phase synchronization between LPFC and MPFC so as to affect the performance of vigilant attention. The result is the tACS of theta band canregulate the EEG phase synchronization in the corresponding frequency band, suggesting that tACS has certain regulation effect on the time-frequency characteristics of EEG and the vigilant attention.