Aimed at the problems that at present prediction and health management are lack in field evaluate methods and technical measurements after the advanced aero planes come into service, a method to evaluate the PHM system fault diagnosis ability based on prior information is proposed. The method confirms the priori point estimate of diagnostic ability index of the LRU and system by using the diagnostic information of the LRU of a large number of equipment accumulated by the PHM working during the aero plane development and test time. Then the posterior distribution is formed by using the field data to modify the priori distribution. At last, the posterior estimate of the diagnostic ability index of the system and LRU can be obtained by the posterior distribution. With the example of percent detected and percent isolated, lower confidence limit of the percent detected and percent isolated under different confidence level are obtained. The result indicates that the method is scientific and reasonable, practical, and can meet the needs of field evaluation under the condition of small model.