The health care domain is a knowledge-intensive domain. The quality of clinical diagnosis depends mainly on the knowledge of health care and clinical experience held by doctors. However, the ability of a single doctor is very limited, so the quality of clinical diagnosis is not high. To this end, this study proposes an aided diagnosis method based on the domain semantic knowledge base. Firstly, based on the knowledge of the medicine subject matter domain in Freebase, a domain semantic knowledge base is established. Then, based on the semantic knowledge base, the algorithms for calculating the weights of the symptoms in the knowledge base, the relevancy of the diseases related to the input symptom set from a patient, and the related symptom set related to the input symptom set from the patient are proposed. Finally, based on the clinical data of 6 kinds of common diseases randomly selected, the method proposed in this study is compared with the existing methods. On the one hand, the evaluation results show that the method of this paper improves the problems and deficiencies of the existing methods. On the other hand, it shows that the method can avoid the “cold start” problem and can quickly support the aided diagnosis of a large number of common diseases. Using the method presented in this paper, it is expected to provide a comprehensive diagnosis service for a large number of common diseases for the general practitioners at the grassroots level, or provide patients with self-diagnosis services for diseases.