In order to precisely study the detection ability under sea fog conditions for shipborne infrared system and to improve the combat efficiency for shipborne defense system under complex conditions, based on a deep investigation for the actual equipment and working principle of infrared system, a "two step method" was put forward to calculate the actual detection distance of the system in the sea fog. Firstly, according to the sea fog intensity level, the theoretical detection distance of infrared sensor was got in advance, and then judge whether the second calculation was needed to get the actual effective detection distance by the detection capability index. The detection capability index model took into account the interactions among the human, marine environment and equipment. The model judges the weight of separate parties reasonably, effectively solves the problem of "computational failure" which is solely dependent on sea fog information.