Abstract: An axial flow fan including an external rotor motor and an impeller. The external rotor motor includes a base, an external rotor assembly, a rotating shaft, and a stator assembly. The base includes a base plate and a sleeve; bearing chambers are respectively disposed at both ends of the sleeve, and bearings are disposed within the bearing chambers. The rotating shaft is rotatably supported by the bearings, and a top end of the rotating shaft is connected to the external rotor assembly. The stator assembly is disposed around the sleeve and is disposed within an inner cavity of the external rotor assembly; an annular protrusion is disposed at an edge of the base plate. A labyrinth seal structure is formed by a complementary engagement of convex and concave structures between the annular protrusion and an end of the external rotor assembly.