The observing failure and feedback instability might happen when the partial sensors of a satellite attitude control system (SACS) go wrong.A fault diagnosis and isolation (FDI) method based on a fault observer is introduced to detect and isolate the fault sensor at first.Based on the FDI result,the object system state-space equation is transformed and divided into a corresponsive triangular canonical form to decouple the normal subsystem from the fault subsystem.And then the KX fault-tolerant observers of the system in different modes are designed and embedded into online monitoring.The outputs of all KX fault-tolerant observers are selected by the control switch process.That can make sense that the SACS is part-observed and in stable when the partial sensors break down.Simulation results demonstrate the effectiveness and superiority of the proposed method.