Abstract: To enhance the thermal dissipation capability of the rotor in a high-speed fractional-slot permanent magnet synchronous motor, an internal rotor circulating liquid cooling system along with its corresponding rotor topology is proposed. Initially, computational fluid dynamics is employed to analyze the flow field distribution within the rotor-integrated circulating liquid cooling system. Based on this analysis, the rheological characteristics of the cooling system and the variation patterns of convective heat transfer coefficients are investigated. Furthermore, the temperature field and temperature gradient of the liquid cooling system under various operating conditions are examined, with a focus on the influence mechanism of contact thermal resistance on cooling performance. Finally, the structural strength, stiffness, and enhanced heat dissipation effectiveness of the rotor are validated through a prototype test platform utilizing a back-to-back loading setup. Both simulation and experimental results demonstrate that the adoption of the internal rotor circulating liquid cooling system significantly reduces the temperature rise of the rotor, thereby increasing the operational speed and power density of the fractional-slot permanent magnet motor.