A stepless speed regulation circuit for driving a motor, comprising: a switch circuit, having an input end, a ground end, a first output end and a second output end, wherein the input end is configured to receive a unidirectional fluctuating signal (Vin) obtained by full-wave rectification of a sinusoidal AC signal (Vsin), the motor is coupled between the first output end (Vol) and the second output end, the switch circuit comprises a first switch, a second switch, a third switch and a fourth switch, wherein the first switch is coupled between the input end and the first output end, the second switch is coupled between the input end and the second output end, the third switch is coupled between the second output end and the ground end, and the fourth switch is coupled between the first output end (Vol) and the ground end; a synchronizing signal generation circuit, configured to generate a synchronizing signal, wherein the synchronizing signal is in a first state at a first half cycle period of the sinusoidal AC signal (Vsin) and is in a second state which is different from the first state at a second half cycle period of the sinusoidal AC signal (Vsin); and a switch driving circuit, configured to select at least one switch from the first switch, the second switch, the third switch and the fourth switch to perform chopping at a first frequency according to the state of the synchronizing signal, and to forma sinusoidal envelope chopped signal (Vo) between the first output end and the second output end, wherein the envelope line of the sinusoidal envelope chopped signal (Vo) is synchronous with the waveform of the sinusoidal AC signal (Vsin), characterized in that the first frequency is 10 times higher than the frequency of the sinusoidal AC signal (Vsin), wherein, when the synchronizing signal is in the first state, the switch driving circuit turns on the first switch, turns off the fourth switch, and performs chopping on the second switch and the third switch in reverse phases; and when the synchronizing signal is in the second state, the switch driving circuit turns on the second switch, turns off the third switch, and performs chopping on the first switch and the fourth switch in reverse phases. a switch circuit, having an input end, a ground end, a first output end and a second output end, wherein the input end is configured to receive a unidirectional fluctuating signal (Vin) obtained by full-wave rectification of a sinusoidal AC signal (Vsin), the motor is coupled between the first output end (Vol) and the second output end, the switch circuit comprises a first switch, a second switch, a third switch and a fourth switch, wherein the first switch is coupled between the input end and the first output end, the second switch is coupled between the input end and the second output end, the third switch is coupled between the second output end and the ground end, and the fourth switch is coupled between the first output end (Vol) and the ground end; a synchronizing signal generation circuit, configured to generate a synchronizing signal, wherein the synchronizing signal is in a first state at a first half cycle period of the sinusoidal AC signal (Vsin) and is in a second state which is different from the first state at a second half cycle period of the sinusoidal AC signal (Vsin); and a switch driving circuit, configured to select at least one switch from the first switch, the second switch, the third switch and the fourth switch to perform chopping at a first frequency according to the state of the synchronizing signal, and to forma sinusoidal envelope chopped signal (Vo) between the first output end and the second output end, wherein the envelope line of the sinusoidal envelope chopped signal (Vo) is synchronous with the waveform of the sinusoidal AC signal (Vsin), characterized in that the first frequency is 10 times higher than the frequency of the sinusoidal AC signal (Vsin), wherein, when the synchronizing signal is in the first state, the switch driving circuit turns on the first switch, turns off the fourth switch, and performs chopping on the second switch and the third switch in reverse phases; and when the synchronizing signal is in the second state, the switch driving circuit turns on the second switch, turns off the third switch, and performs chopping on the first switch and the fourth switch in reverse phases.