The invention provides an optical resonant cavity alternating magnetic field sensing system based on triple resonance and a use method. The system comprises a signal generator, a tunable laser, an attenuator, a polarization controller, an optical fiber taper, a magnetic field sensing unit, a photoelectric detector, a bias device, an oscilloscope, a beam splitter, an electrical spectrum analyzer, a first network analyzer, a second network analyzer, a direct-current magnetic field, a coil and a proportional-integral-differential controller. When the mechanical mode and the ferromagnetic resonance mode of a YIG ball are close to the frequency of the alternating magnetic field to be detected and triple resonance is generated, the system can obtain higher magnetic field detection sensitivity. The system has the advantages of low power consumption, electromagnetic interference resistance and the like, and a new device choice can be provided for the field of magnetic field detection.