A universal input switching power supply has a signal detecting unit, two DC converting units and a physical wiring and controlling unit. The DC converting units are respectively adapted to couple to an AC power source to convert the AC power source to two first DC power sources and changes a voltage of each of the first power sources according to different voltage of the AC power source. The physical wiring and controlling unit is connected to the outputs of the DC converting units and the signal detecting unit and automatically connects the outputs of the PFC circuits in parallel if a high line voltage range of the AC power source is coupled to the full bridge rectifier. On the contrary, the physical wiring and controlling unit automatically connects the outputs of the PFC circuits in serial. Accordingly, the universal input switching power supply has good transmitting efficiency at different AC power source conditions.