A voltage converting controller is applied to a switching voltage converting circuit, in which the voltage converting controller periodically operates a high-side power switch and a low-side power switch in the switching voltage converting circuit with a high-side control signal and a low-side control signal, respectively, so as to convert an input voltage into an output voltage via an inductor. Defining an ideal duty cycle as the rating value of the output voltage divided by the value of the input voltage, when the ideal duty cycle is less than one threshold duty cycle, then the period of the high-side control signal is a constant; and when the ideal duty cycle is greater than the threshold duty cycle, the period of the high-side control signal and the period of the ideal duty cycle are positively correlated.