Aiming at the problem of diagnosis difficulty caused by too many factors of mechanical system, a kind of diagnosing method based on fault phenomenon was presented. The research on mechanical system fault phenomenon space arrived at conclusion that the emergency of each fault phenomenon subject to 0-1 distribution. Therefore, phenomenon vector corresponding to each fault formed cluster whose accumulation point is expectation of vector. After exclusion of abnormal vectors, the distance discrimination was used to fault diagnosis to establish expert system based on fault phenomenon vector. The confirmed result was return back to fault database so that the system achieve self-learning of real-time diagnosis experiences. Finally, the example on X-type hydraulic excavator proves that the diagnostic method has characteristics of good real-time, simple operation and high diagnostic accuracy.