A monitor method of computer system is provided, applying within an interrupt service routine. According to the application of interrupt service, when the interrupt controller sends an interrupt signal to the CPU, the CPU executes a corresponding interrupt service routine based on the interrupt signal, in the meantime, the daemon program generates an entrant code. Before the interrupt service routine stops, the daemon program generates an exit code and saves both the entrant code and the exit code in a storage device. It is benefit for solving the problems occurred in the debugging process according to the entrant code and the exit code of the storage device, and speeding up the process of testing and researching steps.