In this paper, a multiple faults diagnosis (MFD) scheme is proposed based on observer method for satellite attitude control systems (ACS). First, the model of ACS is established. Both the nonlinear and uncertainties of ACS are considered. Then, the multiple faults diagnosis scheme based on observer is proposed. The multiple faults problems in the system level and component level are all considered. In the system level, two nonlinear observers based on analytical redundancy are designed for fault diagnosis of the ACS, which determines the fault source roughly. In the component level, a bank of sliding mode observers are designed to further diagnose the fault of specific actuators in the ACS after having obtained detection results in the system level, which determines the fault source precisely. Finally, numerical simulations on the satellite attitude control system are carried out to illustrate the effectiveness of the proposed scheme.