A stepless motor driving circuit provides a sinusoidal shaped chopping signal for driving a motor. The driving circuit has: a switch circuit having an input which receives a rectified signal rectified from a sinusoidal AC signal and having a first output and a second output where the motor is coupled between; a synchronizing signal generating circuit which generates a synchronizing signal relating to the sinusoidal AC signal; and a switch driving circuit which selects and chops at least one switch in the switch circuit according to the synchronizing signal, and to form the sinusoidal shaped chopping signal which is corresponding to the sinusoidal AC signal.