A motor driving device includes a PWM signal generating unit, a control unit, a driving unit, a floating point selecting unit and a BEMF detecting unit. The PWM signal generating unit generates an input PWM signal having a duty cycle according to a rotation speed command. The control unit generates a driving signal having multiple phases and an output PWM signal. The floating point selection unit selects a floating phase of the motor that is not turned off, and the BEMF detecting unit receives detects the BEMF of the floating phase during ON times or OFF times of the output PWM signal, so as to output a commutation signal in response to zero crossing events occurring in the BEMF. The control unit controls the BEMF detecting unit to detect the BEMF of the floating phase under the ON times or the OFF times of the output PWM signal.