The PHM index is directly affected the design of PHM and the availability of equipment. In response to the shortage of theoretical and implementable methods, a graded demonstration method was introduced, outlining a progression from comprehensive efficiency indicators to PHM comprehensive indicators and then to PHM capability indicators. The availability was selected as the comprehensive efficiency indicator, and the health evaluation rate was defined as the PHM comprehensive indicator. The relationship between availability and health evaluation rate was derived. The optimal health evaluation rate was obtained by maximizing the availability. It was deduced that the health evaluation rate was equal to the product of fault coverage and evaluation accuracy, where depend on the number of sensors and the accuracy of diagnostic/prognostic methods, respectively. This conclusion can guide PHM design. The effectiveness and practicality of this method were verified by cases.