In this paper, a 50 kW stator yokeless modular axial flux motor with strong overload capacity, wide operating speed range and high operating efficiency is designed for the high torque and high speed requirements of the M/G motor in the flywheel energy storage system. Considering the overload capacity and the operating speed domain, three operating conditions of high efficiency, high speed and overload are determined, and the magnetic circuit calculation of the motor and the selection of the pole slot are carried out to determine the motor structure. Considering the influence of the stator material on the motor performance, the motor torque performance and loss characteristics under the three materials of silicon steel sheet, SMC and AMM were compared. A three-dimensional motor electromagnetic field model is established and combined with the finite element method to improve the torque performance. The YASA motor is optimized for the purpose of suppressing losses. After optimization, the maximum efficiency of the motor exceeds 98%, and the peak power density exceeds 5 kW/kg.