Abstract: We consider the problem of attitude stabilization of a flexible spacecraft under complete failure of a single actuator. A novel adaptive observer-based fault-tolerant control scheme is proposed to guarantee the convergence of the angular velocity and attitude of the spacescraft to a bounded region. The spacecraft model under consideration is a flexible body that is almost axisymmetric. A small parameter gives a measure of the non-symmetry of the spacecraft about its axis. Firstly, the system model is simplified by eliminating the coupling term using a homogeneous system approach. Secondly, an adaptive observer is designed to estimate the nonlinear coupling, and the norm boundary of error between the estimated and true values is determined. Based on this, a fault-tolerant control law is further designed to ensure that the system output is input-to-state stable with respect to the estimation of the coupling term. Finally, the simulation results illustrate the efficiency of the theoretical results.