The utility model provides the utility model discloses a soft starting device's of motor detection low -speed back electromotive force zero -crossing detection unit, including the signal isolation modulate circuit, zero passage comparison circuit, signal AD sampling circuit and signal processing unit, zero passage comparison circuit and signal AD sampling circuit are connected with signal processing unit simultaneously, this unit still includes trap, the signal isolation modulate circuit is connected with trap, the signal isolation modulate circuit is connected with trap, trap respectively with zero passage comparison circuit, signal AD sampling circuit connects, trap is with the aliasing in the filtering of back electromotive force's 300Hz high frequency signal. The utility model discloses it is effectual, with low costs to use the unit, interference that the high frequency noise that can significantly reduce brought for the system and the extra calculation expense of bringing for main control unit.