A compound-structure permanent-magnet synchronous machine (CS-PMSM) system used for hybrid electric vehicles (HEVs) enables the internal combustion engine (ICE) to operate at the optimum efficiency region independent of road conditions, thus decreasing emissions and fuel consumption. The key part CS-PMSM is a high integration of two electric machines. The connection ways of the CS-PMSM in the HEV system are discussed and the better connection way is selected based on power. Different topologies of the CS-PMSM are put forward and compared on criteria, such as magnetic coupling, power match, thermal properties, etc. A downsized axial-axial flux prototype machine was designed and manufactured for the testing facilities and concept validation. The performance, magnetic coupling, efficiency, and temperature of the axial-axial flux CS-PMSM prototype are measured and analyzed to provide a reference for this kind of machine.