The present invention relates to a voltage boosting circuit capable of modulating duty cycle automatically, which comprises an inductor, a switching module, and a control circuit. The inductor is coupled to an input for receiving an input power. The switching module is coupled among the inductor, a ground, and an output for switching so that the input power can charge the inductor and produce charged energy, or for switching so that the charged energy of the inductor can discharge to the output and produce an output voltage. The control circuit outputs at least a control signal according to the charged energy and the output voltage for controlling the switching module to switch the inductor and provide the input power to the output, to switch the charged energy of the inductor to discharge to the output, or to switch the input power to charge the inductor.