In view of the lack of scientific and effective diagnosability evaluation index system, a three-dimensional diagnosability evaluation model is constructed from three aspects of fault diagnosis performance, response time and test resources. Ten representative indexes are selected as the elements of the evaluation model to ensure the completeness of the diagnosability evaluation model; A comprehensive evaluation method of diagnosability based on combination weight and grey relation analysis (GRA) is proposed, in which analytic hierarchy process (AHP) and entropy method are used to calculate the subjective weight and objective weight of evaluation indexes. The mathematical programming method is used to integrate the subjective weight and objective weight, and the GRA is used to evaluate the diagnosability of the system. The results show that the method ensures the rationality and scientificity of the diagnosability evaluation results.