This paper investigates the design considerations of a new hybrid excitation synchronous machine (HESM), which is characterized by magnetic shunt rotor and brushless field excitation. First, the basic constraints of flux control capability are discussed and deduced. Then the influence of some key structural parameters on flux control capability of the HESM is investigated by using equivalent magnetic circuits. The influence of permanent magnet width is further studied and verified by 3-D finite element analysis. A prototype HESM with magnetic shunt rotor is designed and developed. The experimental results show the satisfactory output performance, especially the significant sinusoidal EMF waveforms of the machine, and also verify the influence of air-gap variation on flux control capability of the HESM.