The invention belongs to the technical field of electric cars and particularly relates to a power transmission technology. A wheel hub motor driving device comprises a shell (21), a motor, a two-stage planetary reducer, wheel rims (2) and tires (1); and the motor and the two-stage planetary reducer are coaxially arranged, the wheel rims (2) are connected with the output end (5) of the two-stage planetary reducer, and the tires (1) are installed on the wheel rims (2), so that power from the two-stage planetary reducer is transmitted to the tires (1). According to the wheel hub motor driving device, a planetary reducing mechanism is adopted, the manufacturability is good, transmission efficiency is high, the large transmission ratio can be obtained, the rotating speed of the motor can be increased, and thus the power density of the wheel hub motor driving device is improved; output of the reducer serves as a rotating shell and is directly connected with the wheel rims, and wheel detaching and installing are convenient; and brakes are arranged inside the wheels, the outer diameters are large, and thus heat dissipation and thermal capacity guaranteeing are facilitated.