In view of the insufficient design method for rotor bars of cage rotor brushless doublyfed motor, the analytical expression for the rotor current calculation of cage rotor brushless doublyfed motors in steady state was derived starting from analyzing the internal magnetic field of cage rotor brushless doublyfed motor. The iterative method was used to select the most suitable crosssectional area of the rotor bar in consideration of the rotor resistance, which provided a theoretical basis for the rotor design of the cage rotor brushless doublyfed machine. Via finite element simulation, the theoretical derivation was validated. The research results not only help to quickly calculate the rotor current of cage rotor brushless doublyfed motor during design, but also help to optimize the rotor structure design.