A switching power converter has a power converter connected between a DC power source and a load and having a power switch, a current detector detecting a load current, and a PWM controller outputting a first frequency to control the switching frequency of the power switch and outputting an adjustable second frequency to adjust the switching frequency of the power switch according to the load current and a condition of judgment. The condition of judgment serves to determine if the load is a heavy or light load. In the case of light load, the second frequency is reduced to lower the switching loss of the power switch. In the case of heavy load, the second frequency is raised to reduce the ripple of the load current. Accordingly, the conversion efficiency of the switching power converter can be enhanced.