A method based on homography matrix is proposed to remove dynamic sky background,which provides a new approach to detect small target under dynamic sky background.The movement and rotation of satellite can cause the movement and rotation of camera,which have an influence on the imaging of stars,so the homography matrix between different moments was calculated according to the rotation matrix of satellite and the imaging principle of camera on infinity points,and the movement of star background can be removed by this homography.Then,median filter is utilized to suppress earth background.Lastly,three dimensional space-time directional filter combining with Kalman filter is applied to detect and track small targets.The experimental results show that the proposed method can effectively restrain the background,and target positioning accuracy is less than half pixel.