This paper establishes a new mathematics model of fault diagnosis basis of oil spectrometric analysis by means of long time following up the mechanical equipment's lubrication oil. The characteristic parameter of oil spectrometric analysis has been confirmed using a new way that the laboratory experiment study and the mechanical equipment's lubrication oil spectrometric analysis are combined in order to confirm the time of fault and avoid serious fault. It will provide the academic and actual basis for RCM and enhance the dependability. The result of examples proves that the characteristic parameter of oil spectrometric analysis has very high stability and veracity. This method has been proved that it was effective in fault diagnosis basis of oil spectrometric analysis.