The background of fault diagnosis is from the prevention of accidents, and from the reverse progression of accidents to the fault subdivision of the failure stage.By studying the single fault and the coupled fault separately, this paper demonstrates that the current theory of fault diagnosis has been intervened and developed in the fault stage, and it is a rule method of subdividing the fault models into opposite and parallel layers. This method makes the research object a divergent system and cannot be solved.The whole process of laboratory experiments and field failures is tracked, and data analysis proves that failures are dynamic links that build each other in the rotor system, and show the corresponding “homogeneous“characteristics with the change of the “forward” phase of the fault state time, and expand the process layer by layer.Therefore, the application of fault diagnosis theory should be restored to the unified whole and the whole process system, which requires changing the direction of fault diagnosis, reversing the horizon, and mastering the main law of fault development in the unified whole and the whole process based on the "forward" and "homogeneous" methods of time history and fault nature, so as to lay a more solid theoretical basis for intelligent diagnosis.