To gain a deep insight into the development tendency and the key technology of compound-structure electrical machine for hybrid electric vehicles, this paper reviews the compound-structure electrical machine and its key technologies. The composition, operating principle and topologies of different compound-structure electrical machines are analyzed, including the brush and brushless ones. Meanwhile, the advantages and key technology problems of different compound-structure machines are investigated and summarized. The results show that the purely-electrical hybrid schemes based on the compound-structure electrical machine don't have the problems of vibration, noise, wear and regular maintenance, but they also have advantages of simple and compact structure and easy control, compared with the mechanical hybrid schemes based on the planetary gear. Meanwhile, they also show that the investigation of the brushless compound-structure electrical machine must be a development tendency in this field for the brushless compound-structure electrical machines can get rid of brushes and slip rings in the brush ones as well as improve the system stability.