Aiming at the problem of dim target detection in infrared images with sky background,the detection model of infrared targets is analyzed and a detection algorithm was put forward based on the sparse ring decision.The morphological filtering target enhancement method was used for background suppression,then the Constant False Alarm Rate(CFAR) detection method was adopted for image adaptive segmentation to get candidate target points.The Local Self-Similarity (LSS) descriptors of candidate target points were calculated out,and sparse ring was obtained by normalizing the LSS descriptors and partitioning.By means of appropriate criterion,the target point and false alarm points can be distinguished. Experiments show that:1) The algorithm can get ideal results when applied to infrared images with dim and small target against clutter cloud background;and 2) Comparing with moving pipeline filter algorithm,it has lower false alarm rate and is easier for implementation.