A new method is put forward to detect thermal flaw inner cylinder head on-line based on infrared thermograph technology. Aiming at some diesel engine, heat transfer rule for cylinder head is analyzed. Combined with three-dimension temperature distribution, areas in which flaws can be produced are studied and flaw inner cylinder head is machined. Testing platform on detecting flaw is set up and temperature tests on cylinder head are developed. Results show that when there is a flaw inner cylinder head, maximum temperature value of outer surface hindered by flaw will be decreased. On other surfaces, maximum temperature value will be increased, which is accordant with actual heat transfer rule.