The invention relates to a USB serial-port LED double-flash alarm which includes a USB-to-serial-port chip, a USB interface plug-in, LEDs, a current-limiting resistor and a power-supply filtering circuit. The alarm is characterized in that two sending and receiving base pins on the USB-to-serial-port chip form a short circuit. A receiving-state monitoring base pin and a sending-state monitoring base pin on the USB-to-serial-port chip are connected with LEDs respectively. The USB-to-serial-port chip is connected with a host through the USB interface plug-in. In the USB serial-port LED double-flash alarm, the LEDs are driven directly by a host program through a USB interface to flash and alarm and power is supplied by the host directly without an external power supply. The alarm is simple in structure, convenient to install and use and reliable in performance.