A DC-DC converter and a voltage conversion method thereof are provided. The DC-DC converter includes an output unit, a control unit and a trigger unit. The output unit converts an input voltage into an output voltage. The control unit generates a control signal according to the input voltage, an output feedback voltage related to the output voltage and a reference voltage, so as to make the output unit generate the output voltage. The trigger unit generates a first trigger signal according to the control signal, the output feedback voltage and the reference voltage. When the output feedback voltage is less than the reference voltage by a preset voltage, the control unit controls the output unit to perform a voltage-regulating process to the output voltage in advance.