An auto-detection method based on 'cross algorithm' is introduced for infrared image small target(detection.) By utilizing the method on ship infrared search and track system,the algorithm proves to be ability to(detect) the small infrared target rapidly and exactly and with high detection ability.The method includes the following steps.First,an auto detection of sea-sky-line is implemented through Radon transform.Then nearer lines paralleling to sea-sky-line is availed to detect the break point of frequenc...