A non-isolated power supply device and a method for controlling the same, wherein only one DC voltage bus is adopted and the neutral lines of the input voltage and output voltage are directly connected together. During the periods that the voltage polarities of the input voltage and output voltage are not the same, the power supply device is operated in a flyback mode to avoid the generation of an abnormal current. Otherwise, during the periods that the voltage polarities of the input voltage and output voltage are the same, the power supply device is operated in a boost mode to derive a superior efficiency.