The invention discloses a power frequency induction adjustable resonance apparatus and a frequency modulation method thereof for determining a power frequency in-series resonant condition. A frequency conversion control power supply is included. The frequency conversion control power supply supplies voltage to an excitation transformer primary side. An excitation transformer secondary side is connected to a detected test product through a high voltage adjustable reactor. A voltage acquisition unit which is connected to the frequency conversion control power supply is also included. The frequency conversion control power supply gives signals according to acquired voltage signals so as to drive the inductance electric adjusting mechanism to adjust the high voltage adjustable electric reactor, so that the whole system performs resonance under the condition of the power frequency. According to the method, the frequency conversion power supply is adopted. When the electric reactor is adjusted well, a resonant point is searched in a mode of automatic resonance under the condition of low voltage. The resonance condition is determined through comparing the difference between the resonant frequency point and a frequency point of 50 Hz. An adjusting amount and a direction of a reactor air gap are pointed. The resonant condition can be rapidly determined and the power frequency resonance can be rapidly realized. Power frequency resonance boost when the voltage transformer is checked in an on-site mode is provided with convenience. The labor production efficiency is raised and the power frequency induction adjustable apparatus and a frequency modulation method thereof for determining a power frequency in-series resonant condition have a broad application prospect.