The construction environment of metro project is complex; the unforeseeable factors are many; the construction accidents are frequent, and it is difficult to carry out quick and effective emergency decisionmaking and rescue. Firstly, the emergency cases of metro construction are collected; the risk factors are analyzed; and the knowledge ontology model is established by software Protégé. And then the comprehensive similarity between the source case and the target case in the ontology library is calculated; the source case with maximum similarity degree is selected, the key properties of source case in ontology library and target case are compared; and the adaptability of emergency method of similar cases are modified by rule reasoning to improve the pertinence of the emergency method and provide a new method for emergency decisionmaking and rescue of metro accidents. Finally, the rationality of the comprehensive similarity algorithm is verified by a case stady.