In the direct drive waste heat power system, the turbine is directly connected to high-speed permanent magnet generators (HSPMGs), and the transmission efficiency, service life and system volume can be obviously improved by removing the gearbox and screw rod. However, in the design of high-speed electrical machines, a comprehensive multiphysics analysis must be conducted in order to meet various requirements of electromagnetic, mechanical and thermal specifications simultaneously. The design of one HSPMG by the multiphysics approach for the electromagnetic and loss characteristics, rotor stress, rotor dynamics and fluid-thermal parameters, which satisfies all specified multiphysics constraints, is presented. The theoretical results are confirmed by the experimental results on a prototype in terms of electromagnetic, mechanical and thermal characteristics, such as induced voltage, load current, rotor continuous operation and temperature distribution.