Because of its simple and special rotor structure, the cage rotor brushless doubly-fed machine(BDFM) has rich magnetic field harmonic content, which directly affects the rotor′s magnetic field modulation capacity. From the internal magnetic field of the motor, a fast method to calculate the rotor magnetic field of the BDFM is proposed, and the influences of the number and distribution of short circuit rings on the rotor magnetic field are analyzed. Taking the number and span of short circuit rings as the parameters, the MATLAB calculation program is established to analyze the modulation capacity of the rotor. The ultimate modulation capacity of the rotor is studied. The electromagnetic field finite element analysis software Flux is used to establish the motor model for verification. The results are helpful to the optimization design of rotor structure.