Underwater brushless direct-current planetary gear speed reducing motor

The invention relates to the technical field of speed reduction motors, in particular to an underwater brushless direct-current planetary gear speed reduction motor which comprises a motor shell, a stator is arranged in the motor shell, a rotor is arranged in the stator, a rotating shaft is fixedly installed in the rotor, a sensor is arranged at one end of the rotating shaft, and the sensor is connected with the motor shell. When the driving shaft rotates, the water pressure at the second water conveying hole is larger than that at the first water conveying hole, a pressure difference is formed, and therefore under the action of the pressure difference, water flow can enter the second water conveying hole, passes through the heat dissipation holes and is finally discharged out of the motor shell from the first water conveying hole; therefore, when the driving shaft rotates, water flow can circularly flow in the motor shell, and heat generated when the first gear, the second gear and the inner gear ring are me