The invention discloses a low-voltage start circuit used for a BOOST DC-DC converter, and mainly solves the problems of long starting time and high power consumption of the conventional low-voltage starting circuit. The low-voltage start circuit comprises a starting oscillator, a charge pump, an output voltage detector, a starting power tube, a first gate driver, a second gate driver, a main switching tube and a rectifying tube; the starting oscillator, the charge pump and the starting power tube are used for completing initial starting; and the first gate driver and the second gate driver are controlled by the output end of the output voltage detector to be switched between a starting state and a gate driving state to realize circuit multiplexing, and to complete accelerated starting along with the main switching tube and the rectifying tube. By virtue of the low-voltage start circuit, the starting speed can be accelerated and the starting time can be shortened; meanwhile, by adoption of a circuit multiplexing technology, the circuit area is reduced effectively and the circuit design complexity and power loss can be lowered; and the low-voltage start circuit can be used for the BOOST DC-DC converter controlled by a lagging current model.