Due to failure of mechanisms to deploy, some large communication satellites lost their ability and resulted in huge economic cost. A space robotic system is expected to perform the on-orbit repairing mission. It is a tremendous challenge to navigate a space robot in final approach since the targets are generally non-cooperative. Rectangle features, which are common in the configuration of a satellite, can be chosen as the recognized objects. However, these characters are very large. Limited by the FOV (field of view), a monocular camera can not supply enough information of the rectangles. In this paper, a method based on monocular camera is proposed to determine the attitude of a large non-cooperative target using a partial rectangle. Firstly, the relationship of a rectangle and circular points by camera is derived and fused. Secondly, the attitude measurement algorithm is acquired from the constraint and a virtual rectangle is reconstructed. Lastly, the algorithm is verified by mathematic simulations which are very close to reality. The results show the validity and flexibility of the proposed method.