A method of controlling a buck-boost converting circuit is provided. The buck-boost converting circuit has an inductive element, a first conduction controlling element, a second conduction controlling element, a third conduction controlling element, and a fourth conduction controlling element. The method includes: controlling the first and third conduction controlling elements to be electrically conductive and the second and fourth conduction controlling elements to be electrically nonconductive according to a first duty setting; determining a second duty setting whose generation is independent of that of the first duty setting; controlling the first and fourth conduction controlling elements to be electrically conductive and the second and third conduction controlling element to be electrically nonconductive according to the second duty setting; and controlling the second and fourth conduction controlling elements to be electrically conductive and the first and third conduction controlling elements to be electrically nonconductive according to a third duty setting.