This paper presents a scheme of fault diagnosis for flexible satellites during orbit maneuver.The main contribution of the paper is related to the design of the nonlinear input observer which can avoid false alarm arising from the disturbance from orbit control force.The effects of orbit control force on the fault diagnosis system for satellite attitude control systems,including the disturbing torque caused by the misalignments and the model uncertainty caused by the fuel consumed,are discussed,where standard Luenberger observer cannot work well.Then the nonlinear unknown input observer is proposed to decouple faults from disturbance.Besides,a linear matrix inequality approach is adopted to reduce the effect of nonlinear part and model uncertainties on the observer.The numerical and semi-physical simulation demonstrates the effectiveness of the proposed observer for the fault diagnosis system of the satellite during orbit maneuver.