Abstract: Series resonance is one of the resonance types of medium-frequency power supply.Here in the article the basic principle of the medium frequency power supply circuit is described，and the working characteristics of series resonance and those of parallel resonance are comparatively analyzed.The obvious energy-saving effectiveness of series resonance method is proved with actual application examples thereof in steel pipe heating process.It is concluded that using series resonance medium frequency power supply for steel pipe heating can bring about significant energy-saving effectiveness.