Variable-flux machine (VFM) is designed to solve the problems of energy crisis and environmental pollution for traditional automotive industry, which satisfies the requirements of high-efficiency operation of new energy vehicle driving machines in a wide speed range. The development of VFM system and its key technologies were reviewed. First, the application background and operating principle of VFMs were introduced. Then, the topologies, advantages, and disadvantages of various VFMs (including VFMs using AC flux-adjustment coils, DC flux-adjustment coils, and mechanical structures) were analyzed, and the structure characteristics and design methods of different VFMs were summarized. Finally, in view of the special working principle of VFMs and the requirements of on-line parameter identification, flux-adjusting current control, and torque ripple suppression during load and on-line flux-adjustment operations, the control strategies specific to VFMs (including flux observer, current and torque control strategies, flux-adjustment, and speed regulation strategies) were summarized. The theoretical method, implementation approach, system structure, advantages, and disadvantages of various control strategies of VFMs were analyzed. Results show that the air-gap magnetic field of VFMs could be adjusted to expand the speed range and improve the efficiency of machines in high-speed range. By adopting advanced control methods, the on-line flux-adjustment and speed regulation could be realized, and the system could operate efficiently and steadily with torque ripple and flux-adjusting loss reduced. Thus, with the development of VFM topologies and related technologies, VFMs have great application prospects in the fields of electric vehicles and numerically controlled machines, and will become one of the key technologies in the future development of new energy fields.