The invention discloses a PWM / PFM dual mode automatically-switched step-down DC-DC converter, comprising a conversion circuit and a control circuit, wherein the control circuit comprises an output voltage sampling circuit, a PWM control module, a control logic module and a driving module; two output ends of the driving module are connected with the control end of a main switch tube and the control end of a freewheeling switch tube; the PWM control module comprises an error amplifier, a comparator, an oscillator and a sawtooth wave generation circuit; the first input end of the error amplifier is connected with the output end of the output voltage sampling circuit, and the second input end of the error amplifier is connected with the reference voltage; the first input end of the comparator is connected with the output end of the error amplifier, and the second input end of the comparator is connected with the output end of the sawtooth wave generation circuit; the output end of the comparator is connected with the first input end of the control logic module, and the output end of the oscillator is connected with the second input end of the control logic module; and the output end of the control logic module is connected with the input end of the driving module. In summary, the PWM / PFM dual mode automatically-switched step-down DC-DC converter disclosed by the invention determines the switching points by using the delay of the comparator, and is simple in structure and small in occupied chip area.