To solve the problem of high false alarm and low real time for aerial video with rotational ground background in moving object detection, a real time algorithm based on optical flow method is proposed. The image feature points are detected first, the optical flows are calculated, and the motion vector of the image background is estimated through optical flow field. Then the target size is calculated according to the flight height and the image is divided into blocks. Each feature point optical flow is compared with the motion vector of the image background to find out the candidates of the target feature point. Finally the candidates is added up in each block and the target areas are determined. Moving objects detection experiment is achieved for 360 pix×432 pix central area of two groups of experiment video. The result indicates that the algorithm can exactly detect the ground moving object with a low false alarm. The average consuming time is 29.460 ms/frame and 31.505 ms/frame, thus satisfying the real time requirements.