Abstract: Flywheels are main actuators of satellite attitude control systems, and fault detection and recovery of flywheels is significant on keeping the normal satellite attitude. A high fidelity open-loop system of flywheel was modeled and the second order nonlinear continuous extended state observer(ESO) was established via the nonlinear model. Under the assumption that the system external disturbances are minor and are neglected, fault in flywheel was regarded as the main system external disturbance. Therefore, by using ESO, not only the system state was obtained, but also the fault was estimated with exactness and swiftness. With the fault obtained, fault detection was accomplished without generating system residual, and fault recovery was also achieved by compensating the system input voltage, which ensures the normal flywheel speed and satellite attitude. The simulation results demonstrate the validity of this scheme.