The present invention provides a method for preventing current overloading and saturation of a switch power supply, including one of the steps of checking whether a primary current of an transformer, and a current of an induction or a current of field effect transistor being excess an upper limit current; and a step for generating an adjusting signal so as to directly or indirectly adjust an error signal if the upper limit current is excess the upper limit, so that during subsequent pulse adjustable periods, a duty cycle is reduced, the primary current or the induction current or field effect transistor peak current value are reduced.