A liquid-cooled motor device includes a motor casing, a motor, a coolant casing and an impeller. The motor casing receives the motor, and defines a cooling space surrounding the retaining space, a coolant intake hole, and a coolant draining hole. The coolant casing defines a drawing space, a coolant inlet and a coolant outlet. The impeller is disposed in the drawing space and connected to an output shaft of the motor, and is rotated by the output shaft to draw a coolant into the drawing space via the coolant inlet, and to force the coolant into the cooling space via the coolant outlet and the coolant intake hole, such that the coolant removes heat generated by the motor, and is expelled from the cooling space via the coolant draining hole.