This paper presents the structure of a knowledge-based ECG automaticdiagnosis system and the key technologies applied in the system. Using database technology to store, the ECG knowledge-database is constituted of ECG knowledgeand a set of rules .The inference engine of the system simulates the real diagnosis of the experts and combines the forward reasoning with inexact reasoning. At the same time, complicated reasoning is avoided. Because the inference engine is independent of what is stored in the ECG knowledge-database, the system is suitable for expert system.