Abstract: Working principle of brushless doubly-fed machine and characteristics of different kinds of rotor structures were introduced, and brushless doubly-fed machines with four kinds of rotor structures were modeled and simulated by Ansoft software including reluctance rotor, cage rotor, salient poles rotor and salient poles rotor with bars. The results of distribution of magnetic force lines and frequency spectrum of air-gap magnetic field harmonic of the brushless doubly-fed machines were gotten through 2D transient electromagnetic field finite element, which indicate effects of rotor structures on magnetic field modulation of brushless doubly-fed machines. The final conclusion was the reluctance rotor has the best effect on magnetic field modulation and the cage rotor has the worst effect, which provides foundation for optimal design of rotor structure of brushless doubly-fed machine.