A computer power saving system includes a computer, an UPS, a power detecting device, and an USB device. The computer includes a USB port and a power management module, and can work in a normal mode or in a STR mode. The UPS is connected to the computer and used for supplying power to the computer upon a condition that a commercial power supply stops supplying power to the computer. The power detecting device detects the state of the commercial power supply and sending out an abnormal power signal when the commercial power supply stops supplying power to the computer. The USB device sends the abnormal power signal to the USB port. When the computer works in the normal mode, the power management module detects the USB port, and controls the computer to shift to the STR mode if the USB port receives the abnormal power signal.