The invention has disclosed a system power management circuit comprising: a printed circuit board and a hardware monitor. The printed circuit board includes at least a first power connector, a second power connector, and more than one detection circuits disposed thereon; wherein the first power connector is used for electrically connecting a power supply, the second power connector is used for electrically connecting a power connector of a motherboard, inputs of the detection circuits are electrically connected to the first power connector, respectively. The hardware monitor is electrically connected to outputs of the detection circuits, and used for converting electrical signals outputted from the outputs of the detection circuits into corresponding digital signals, as well as for transmitting the digital signals to the motherboard via a two-wire bus, the two-wire bus is bi-directional, and may be an I2C or a SM bus.