As for the abnormal behaviors of tri-rotor UAV's actuator,a Two-Stage Kalman Filter (TSKF) based diagnosis algorithm is designed.Firstly,the dynamics of the tri-rotor UAV under its actuator's abnormal behaviors are analyzed and then approximated under the hovering state to obtain the system state space equation.Secondly,the abnormal behavior estimation sub-filter is designed based on the Kalman filter to obtain the optimal estimation of the abnormal behaviors of the UAV's actuator.The coupling equation is introduced to put the optimal estimation into the state estimation sub-filter to obtain the optimal estimation of state errors.The abnormal behaviors are isolated from the system states,so as to diagnose the actuator's abnormal behaviors.Finally,numerical simulations are implemented and the results show that the proposed algorithm can well diagnose the actuator's abnormal behaviors.