Infrared focal plane array rotation search system catches attention because of simple system structure, high flexibility and low cost compared with the linear array search system. The dim-small target detecting technology under complex background is a difficult subject in infrared focal plane array rotation search system. In order to effectively suppress background noise and reduce false alarm caused by complex background, constant false alarm rate algorithm based on weighted feature of linear target was presented aiming at the research of imaging properties of infrared focal plane array rotation system. Firstly, the discrete potential target image was obtained initially by background suppression and threshold segmentation. Next, three characteristics of linear target were analyzed and the total weighted value was calculated by quantifying and weighting. Finally, the weighted value was mapped to the discrete target image and the image sequence about the weighted value was achieved, then a new segmentation threshold obtained by CFAR algorithm was applied to suppress the background interference and reduce false alarm and false positives. The experiment results show that the proposed method can significantly reduce the false alarm caused by the complex background in infrared focal plane array rotation search system and can ensure to detect the dim-small target.