Vacuum poloidal magnetic field of tokamak in alternating-current (AC) operation isinvestigated.It is found that the vacuum magnetic field in AC operation is qualitatively differentfrom that in direct-current (DC) operation.In the DC case,the vacuum magnetic field variesalong the poloidal direction with one period,while in the AC case,the vacuum magnetic fieldvaries along the poloidal direction with two periods.This implies that two sets of vertical fieldcoil may be needed for the AC operation. Vacuum poloidal magnetic field of tokamak in alternating-current (AC) operation is investigated. It is found that the vacuum magnetic field in AC operation is qualitatively different from that in direct-current (DC) operation. In DC case, the vacuum magnetic field variesalong the poloidal direction with one period, while in the AC case, the vacuum magnetic field varies along the poloidal direction with two periods. This implies that two sets of vertical field coiles may be needed for the AC operation.