The AC magnetic susceptibility measuring device is composed of three parts: phase-locked amplifier, phase-locked waveform generator and adjustable rate heating furnace. The magnetic susceptibility and loss of the sample can be measured from room temperature to 500℃. The temperature at which a material can change between a ferromagnet and a paramagnet, i.e. the transition temperature at which a ferromagnet changes from a ferromagnetic phase to a paramagnetic phase. It can also be said that the second order phase transition occurs at the transition temperature. Below the Curie point temperature, the material becomes a ferromagnet, and the magnetic field associated with the material is difficult to change. When the temperature is higher than the Curie point temperature, the substance becomes a paramagnet, and the magnetic field of the magnet is easily changed by changes in the surrounding magnetic field.