The present invention discloses a high-speed motor, which comprises a motor casing, a motor main shaft, a motor stator and a motor rotor, wherein the motor stator is sleeved in the motor casing; the motor main shaft is arranged in the motor rotor and comprises a front half shaft, a rear half shaft and a pull rod; the pull rod is freely sleeved in an inner hole of the motor rotor; the front end of the pull rod is in threaded connection with the front half shaft; the rear end of the pull rod is in threaded connection with the rear half shaft; the front half shaft is tensioned with the rear half shaft by the pull rod; and the motor rotor is clamped between the front half shaft and the rear half shaft. The high-speed motor has the advantages of simple structure, convenient disassembly and assembly, convenient processing, large transmission torque and high critical rotation speed. The invention also discloses an assembly method of the high-speed motor.