The debugging method and tools of fault tolerant computing systems are explored. The presented approach is based on use of RT scope multitask debugging tools at the operating system level on a prototype of the system considered. The paper first discusses requirement on environment during developing fault tolerant computer. The system architecture of debugger for fault tolerant computer is presented. The driver is designed to perform real time communicationn function.The cross debugging environment of real-time multitasks is set up under Windows. The results show that it greatly decreases the complexity of computer system and brings much usefulness into developing and testing.