The high speed spindle motor is the crucial component of motorized spindle. The heating problem of motor directly affects the machining accuracy and operation reliability of motorized spindle. Taking a 100 000 r/min spindle asynchronous motor as an example, the thermal analysis and efficient cooling structure design are carried out to ensure the reliable and stable operation of the spindle motor. The main heat source of the spindle motor is determined and the influence of rotor speed and rotor surface roughness on the air friction loss of the spindle motor is studied. Considering the influence of rotating magnetic field and harmonics on the calculation of iron loss, the teeth and yokes of the stator are separately modelled in the finite element method to improve the simulation accuracy and analyze the distribution law of motor iron loss. The 3D thermal simulation of the motor is carried out based on hydrodynamics, and an efficient cooling structure design scheme is determined by comparing the cooling effects of the mainstream circumferential spiral and axial Z-shaped cooling structures. Furthermore, a design scheme of opening air slots in the rotor core is proposed, which increases the thermal resistance between the rotor core and the shaft and improves the heat dissipation capacity of the motor to keep the temperature rise of the rotating shaft within a safe range. Finally, the mechanical strength of the rotor structure is checked to ensure the reliable operation of the spindle motor.