Abstract: Fault detection and diagnosis of closed loop control system is one of the problems in urgent need of solving in satellite control engineering. Fault of one actuator in closed loop system will affect the outputs of other normal actuators because of fault propagation. Flywheels are main actuators of satellite control system in long term orbitmove mode and faults are unavoidable owing to their continuous mechanical movement. The fault would have a strong impact on the satellite if it can not be detected and isolated in time. This paper presented a fault detect and isolate method based on the direct redundancy of flywheels, its basic idea is that there exists consistency of the flywheels′ outputs (change rate of rotation rate) in redundant configuration flywheels system, and inconsistency in the measured rotation rates is then ametric that can be used for detecting faults and subsequently for fault isolation. This method didn′t need to integrate the torque control command of every flywheel and reduced calculation amount. Simulation results showed that this method could detect and isolate incipient fault in real time. At last we made optimizing configuration analysis of the direct redundant flywheels from the fault detect performance point of view.