Abstract: A computer mainframe signal monitoring system connected to the mainframe of computer to monitor its signals, including a main control circuit, a data catching and processing integrated circuit and a monitor respectively connected to said main control circuit and controlled by it, wherein the data catching and processing integrated circuit can be mounted in an interface card or the mother board to catch the major function signals of the mother board such as address bus signal, data bus signal, control bus signal, and regular I/O signals, then to send the signals to the monitor; the monitor is comprised of a LED array and a driver IC, the driver IC turns on the respective LEDs of the LED array upon receipt of signals from the data catching and processing integrated circuit, so that the operator can monitor the actual operational conditions of the internal hardware circuits of the computer through the LEDs of the LED array.