A three dimensional finite element mathematical model of electromagnetic field and temperature field was developed to investigate the effect of high frequency amplitude-modulated magnetic field on the billet surface temperature. The results show that the induction heat power of magnetic field amplitude-modulated with sine wave and square wave is unrelated to the frequency of modulating wave. The induction heat power of magnetic field modulated with square wave is between the induction heat powers of the two constant amplitude magnetic fields employed to modulate. If the induction heat powers of the two constant amplitude magnetic fields are determined, the induction heat power of magnetic fields modulated with square wave is determined by the relative scale of the acting time of the two constant amplitude magnetic fields. The induction heat power of magnetic field modulated with sine wave is about 62-64% of that of the high frequency magnetic field before modulation