The present disclosure relates to an apparatus and method for initializing a DC-DC converter, and can stabilize an output voltage in an initial state, and prevent malfunction of a system control through initialization and high-efficiency DC-DC converters, by converting a variable voltage into a predetermined constant voltage and generating a reset signal for initializing a constant-voltage circuit so as to initialize the DC-DC converter and a control unit composed of the constant-voltage circuit, if the converted voltage reaches a target voltage, in an initialization DC-DC interval, and then converting the variable voltage into the predetermined constant voltage and outputting the converted voltage, in a high-efficiency DC-DC interval.