To satisfy the requirement of detecting small targets in infrared image, an infrared targets detecting arithmetic is put forward based on Scale Invariant Feature Transform（SIFT）and weighted entropy. The method considered the imaging character of small targets. SIFT feature is used to extract special points and the possible location of target is obtained through the match between frames. Weighted information entropy is applied to find feature of gray scale information and entropy. Then potential targets are judged again by the features which have been calculated for the need of real-time dim small targets detection in heavy cloud clutter and complex infrared backgrounds. Experiment indicates that the method has an advanced practicability, and the performance of robustness is good in the sky background.