To calculate the centre of small light points in images, an effective centre arithmetic which combines morphology with optical flow estimation is proposed. After dilating image sequences, three consecutive frames of the image are operated by using optical flow equation, then they are accumulated to enhance the small object energy in the images. This arithmetic makes the classical centroid method reach subpixel precision when it processes this kind of images. By error analysis and experiments, it can solve the problem of small target centre determination.