The problem of actuator fault diagnosis of the quadrotor unmanned aerial vehicle （UAV） attitude control system is studied， in which the nonlinearity of the model and the external disturbance during the flight of the quadrotor UAVs are both considered. Thus disturbance and error can be handled as unknown inputs into unknown input observer， which is insentitive to unknown inputs but sentitive to faults. A new kind of unknown input observer is designed for fault diagnosis. At the same time， in order to maintain the stability of the quadrotor UAV after the failure occurs or still move according to the predetermined trajectory， we design a model reference fault-tolerant controller for the attitude control system of the quadrotor UAV after obtaining the accurate fault estimation， so that it can follow the expected trajectory. Finally， the simulation results validate the proposed fault diagnosis and fault-tolerant control method.