A modulation circuit for voltage to duty-cycle conversion is provided. A first input end and a second input end of a comparator are supplied with a first voltage and a second voltage via a first switch and a second switch respectively. An output end of the comparator outputs a comparison result signal. A charging end of a charging capacitor is connected with a charging current source and a grounding reset module, and is connected with the first input end via a third switch, and is connected with the second input end via a fourth switch. When the comparison result signal flips over, a control module controls the grounding reset module to switch an on-off state of a first switch group including the first switch and the fourth switch and a second switch group including the second switch and the third switch.