Fault diagnosis for sys tems with many possible fault types and many fault symptoms are considered. The fault causesymptom matrix is established for showing the complicated relation ship between the fault causes and the symptoms. The fault diagnosis based on the matrix mainly includes two steps: the first step is using the fault ca usesymptom matrix sieving method to reduce the solution space； and the secon d is using the geometric distance distinguishing approach to determine the order o f th e possibility of fault cause. And the design of a fault diagnosis expert system based on the fault causesymptom matrix is also discussed.