With the evolvement of computer technology, both software systems and hardware systems have been developed into large-scale and sophisticated infrastructure. To test these systems, traditional static analysis methods have proved inefficient. This paper introduces a method based on on-line monitoring. It uses the monitor system MS-1 which is a distributed event-driven hybrid monitor system with synchronous clock system. On-line monitor program OM combines the control of the monitor system, collection of event trace and simple analysis tools to fulfil the on-line monitor. OM offers the internal states and the dynamic behavior giving the help to debug and test the computer systems, both for hardware and software. Our experience proved that this method can alleviate the difficulties of debugging and testing of parallel and distributed computers efficiently.