Disclosed is a power supply unit provided with AC/DC voltage detection and a power supply system incorporating such power supply unit. The power control unit of the power supply unit can manipulate the first stage power circuit and the bypass switch thereof according to the form and magnitude of the first voltage which is inputted to the power supply unit in order to boost the overall operating efficiency. When the inventive power supply unit is applied to a high-level DC power supply system, the problem that the power supply unit can not output a rated output voltage due to an insufficient DC first voltage can be avoided.