Lucky imaging technology is widely applied in astronomical imaging system because of its low cost and good performance. However, the probability of capturing an excellent lucky image is low, especially for a large aperture telescope. Thus a method of adaptive image partition is proposed in this paper to extract any lucky part of an image so as to increase the probability of obtaining the lucky image. This system is comprised of a telescope and three cameras running synchronously at the image plane, the front defocus plane and the back defocus plane respectively. Two out-focus cameras have the same defocus distance. Our algorithm of selecting each lucky part of the space object picture, which is influenced little by atmosphere turbulence, is based on the difference between pictures obtained by the front and the back defocus cameras. Then image stitching is used to obtain the entire sharp picture.