A novel air leak diagnosis and localization method for vessels is proposed. The tempreture field around the leak changes during air inflation and deflation.The changing phenomenon is acquired by thermal camera and the best detecting time is confirmed by temperature curves.Then a local gray-entropy difference algorithm is used to identify the leak area from infrared images captured during inflation and deflation. The gray information of local gray-entropy enhances the difference between leak area and non-leak area largely meanwhile the entropy information of local gray-entropy improves robustness performance.Experiments verify that the leak localization method is effective and sensitive.