The rotor structure is one of the key factors affecting the performance of the brushless doubly fed machine (BDFM). This paper presents a "double-sine" wound rotor for the brushless doubly fed generator to reduce the harmonic contents of the rotor winding. Ideally, so-called double sine is to make the magnetomotive force (MMF) space vector in one set of three-phase symmetric windings only containing two sinusoidal-waveform magnetic fields of different pole pairs. According to the principles of tooth harmonic and sinusoidal winding, the double-sine winding is designed to be constituted by double-layer unequal-turn coils. The design procedure and rules are given in detailed with the design example. The steady-state performance of a 60 kW prototype generator with double-sine wound rotor is analyzed by the simulation and experimental test, and the validity of the design method for proposed rotor structure is verified.