Abstract: In this paper, a double loops controller design method is provided in accordancewith the characteristics of large flexible spacecraft, such as low frequency, low damping and coupling between rigid body and flexible appendages. Optimal sliding-modecontrol method is used, and robust stability is considered. Finallyg numerical simulations show that vibration suppression and accurate pointing are accomplishedby using two separate control loops with certain robust property. Based on modetruncation modle of the flexible spacecraft.