Step scan is a way to realize large pitch region search in infrared search system,which is used to detect incoming targets in a large altitude range.As the pitch baseline needs to be constantly adjusted during the stepping process,the overlap between the pitch search range and the target altitude is accidental,and its search ability is difficult to be evaluated by field tests.Aiming at the infrared search system adopting step by step scanning mode,the law that the target meets the instantaneous visual field of the search system is analyzed in this paper.Based on the encounter probability model of the target and the instantaneous field of view of the infrared search system with a fixed baseline,the encounter probability model of the infrared search system under step by step scanning mode is further given.Taking an infrared search system as an example,the mode is used to compare the search effects of two step by step scanning modes and to analyze the influence of different search airspace ranges on the operating distance and corresponding encounter probability under step by step scanning mode,providing a reference method that can be used to evaluate the search ability of this type of infrared search system and to maximize its performance.