The invention discloses a soft start control method of LLC full-bridge converter based on variable duty cycle. The method is to control four MOSFETs on the primary side of the LLC full-bridge converter, as that signal of the upper and lower switch tube of the same bridge arm are complementary, at that same time, the duty cycle of the switch device is adjusted accord to the start-up time tr obtained according to the output voltage, so that the duty cycle of the driving signal of the switch device is gradually increased to a specified value according to a specified curve, so that the output voltage rises from 0 to a given voltage to reach a soft start. The method of the invention weakens the current impact in the start-up process of the traditional LLC full-bridge converter, reduces the output voltage overshoot, makes the circuit start more stable and reliable, and has higher engineering use value. The control method is reliable, the circuit is simple and easy to realize.