Abstract: The general airgap field modulation theory normalizes a unit machine into a cascade of three basic elements, namely the excitation source, modulator and filter, analyzing almost all electric machines with different types and principles in a unified framework, offering a brand-new perspective and powerful tool for performance analysis and topology innovation, becoming one of the research hotspots in electrical engineering in recent years. This paper retrospected the historical development of electric machine theory, and briefly introduced the concept of the magnetic field modulation and basic framework of the general airgap field modulation theory and the attempts were made to further refine it. Detailed examples of how to use the developed theory to qualitatively analyze the operating principle, torque production mechanism and magnetic field modulation behaviors, to quantitatively calculate the electromagnetic performances like torque characters and losses, to make innovations from topologies of typical electric machines, and to optimize parameters under theoretical guidance were reviewed. Finally, the future researches and trends were prospected.