By analyzing the influence of various infrared detection factors on infrared image taking experimentally, a temperature compensation method is used to correct infrared images on the basis of the position control chart of infrared image collecting points. Then, the collected infrared images are segmented according to the actual operation characteristics of diesel engines. Finally, an area comparison method is used to discriminate and diagnose the faults in the diesel engine on the basis of the segmented infrared images. The experimental result shows that by adjusting the proportion and parameters of infrared images, the requirement of diagnosing most heating faults in diesel engines can be satisfied.