In connection with the infrared target detecting under complex cloud backgrounds,a small target detection algorithm based on the optical flow estimation and adaptive was put forward.Fistly,the infrared image under cloud backgrounds was analyzed based on the optical flow and the cloud movement was extracted.The traditional gradientbased optical flow was proved based on the characteristics of cloud movement and the two constraints of the optical flow equation in the calculation of optical flow.And,it was found that the cloud movement had a greater impact on the target detection,the algorithm of adaptive suppress the complex backgrounds was introduced to reduce the interference of the cloud.the background factors on behalf of background complexity was used to adjust the threshold in the cloud region extracted in the optical flow analysis.In this way,the background suppresion was produced in the cloud movements.It would easily cause false alarm and the algorithm simply the calculation,and reduce the influence of the cloud and the false alarm.Experimental results show that this algorithm can detect small targets and significantly reduce the false alarm caused by the cloud area.