Under the power supply mode of frequency converter, high speed permanent magnet motor has large rotor eddy current loss. Due to the poor heat dissipation capacity of the rotor, it is easy to cause high temperature rise of permanent magnet and irreversible demagnetization. The shell water cooling structure can effectively remove the heat from the stator side of the motor, but the cooling effect of the water cooling structure is limited for the rotor of high speed permanent magnet motor. Taking a 15 kW, 30 000 r/min high speed permanent magnet motor as an example, an air-water hybrid cooling structure is designed. Based on the fluid structure coupling calculation method, the effects of water velocity, wind direction and different cross-sectional area of air duct on the temperature rise of motor permanent magnet are analyzed, and the relative optimal values are obtained. Compared with the water cooling structure only, the air-water hybrid cooling structure can reduce the temperature rise of permanent magnet by 18.1 K. This structure can provide a certain reference for the cooling system design of high power high speed permanent magnet motor.