The invention relates to a structure which can raise the protection grade of an external rotor motor. A stator and a stator coil are provided on a main shaft. Bearings are provided at the two ends inside the main shaft. A motor rotor is provided outside the stator coil. The motor rotor is provided on the main shaft through the bearings. One end of the shaft is in a bolt joint with a fasten bolt. An end cap is provided on the main shaft through the fasten bolt. A leading-out wire is provided on the end cap. The end cap and the motor rotor cooperate to contain the stator coil. The end plane of the motor rotor is provided with a rotor tongue and groove arranged in a circumferential manner. The end plane of the end cap is provided with an end cap tongue and groove arranged in a circumferential manner. The rotor tongue and groove and the end cap tongue and groove splice with each other to cooperate. A gap exists between the rotor tongue and groove and the end plane of the rotor and another gap exists between the end cap tongue and groove and the end plane of the motor rotor, such that a labyrinth-shaped slit is formed between the transverse plane of the motor rotor and the transverse plane of the end cap. According to the invention, teeth mutually plugging into each other exist between the motor rotor and the end cap. The labyrinth structure formed by the motor rotor and the end cap can prevent water coming from different directions from intruding and the motor can reach a protection grade of IP54.