In connection with the infrared target detecting under complex backgrounds,a detecting algorithm based on symmetrical displaced frame difference (DFD) and optical flow estimation is put forward. Firstly,through using symmetrical DFD algorithm on infrared sequence frame,possible target areas are extracted by using frame difference (FD) algorithm and auto-adaptive segmentation of threshold. The area is extended and piled up,then the possible target area is gotten in continuous frames. Secondly,every infrared frame′s optical flow field is calculated,and through threshold segmenting and combined mathematical morphology algorithm targets are detected in areas. This method can sufficiently utilize the advantages of little calculation of symmetrical DFD algorithm and high exactness of optical flow detecting method. So it can cut down the calculation as well as satisfy the veracity. The experimental result shows that this method works perfectly and can effectively detect infrared targets under complex backgrounds.