An air leakage localization method based on infrared image fusion was investigated. Modeling and simulation of heat transfer coupled with steady air-leak flow were addressed by the finite element method. The temperature difference in leak throttling process was derived from the expression of Joule-Thomson coefficient. In the experiments, metallic and metalloid test objects were charged with cryogenic compressed air respectively, which enhanced the phenomenon, and their temperature fields were acquired by an infrared camera. Leak diagnostic criterion was set as different values referring to the thermal conductivities of various tested materials, and entropy fusion operation among the infrared images captured before and after inflation was conducted. Then, the leak positions were located by a certain strategy. The advantages of the method were demonstrated by experiment, especially in eliminating the interference of surface defects and the influence of shapes by employing the entropy fusion algorithm.