Abstract: In this paper,some fundamentals of fault diagnosis technique for dynamic systems are proposed,then the common limitations of all kinds of traditional diagnosismethods,which are only based on data processing technique,are pointed out,andfurthermore,the principle and advantages of expert system used for system faultdiagnosis are studied。As an example of effective application,SCSD,a fault diagnosis expert systemof S.C.R.D.C.speed control device,which was developed by us,is especiallygiven.It contains the summary,functions,characteristics of SCSD and results of aseries of analog fault diagnosis tests to SCSD.Finally some application prospects of expert system,as a new method of system fault diagnosis,are pointed out.