In order to improve the recognizing ability of the infrared objectives under the island's background, the recognition method is proposed based on average gray. While the infrared detector recognizes a ship as a target at the long distance, it's too difficult to identify it from the surrounding near the ship and the islands which have the exterior contour similar to the ship. Through researching the islands and the ships' component, comparing their infrared characteristics and as well as getting the data from the IR image, it is found that the average gray of the ship is higher than the island's average gray.