According to the DC-DC conversion circuit and conversion method, variation of an input voltage is reflected by detecting a voltage at a secondary winding of a transformer, a reference voltage is adjusted by using the detected input voltage signal, a feedback voltage signal is compared with the reference voltage, a duty cycle control signal is adjusted according to a comparison result, and conduction and shutting-down of switching transistors are controlled according to the duty cycle control signal to adjust an output voltage of the DC-DC conversion circuit, so as to enable the output voltage to vary with the input voltage.