The invention mainly relates to the field of driving of luminescent devices, and provides a dimming driving circuit for driving LED in a voltage conversion power supply system based on alternating current electric supply and a dimming method. Duty ratio of a PMW driving signal is linearly changed along with change of phase corner cut of alternating current. A dimming unit is used for detecting change conditions of a conduction angle of the alternating current, and meanwhile, the duty ratio of a modulating signal is further controlled to be changed along with trend of degree change of the conduction angle.