Abstract: An online fault diagnosis method is proposed to make sure that the aircraft flight is safe as the failure occurs in the control surface, and the evaluation of the flight performance can be made based on the fault diagnosis information. Firstly, in order to overcome the shortcoming of the filtering performance of the UKF (unscented kalman filter), which will be decreased as the system state has higher dimensions, a fault diagnosis method based on the cubature Kalman filter algorithm is designed. The problem that the filtering effect of the UKF with a high dimensional state is not ideal can be solved by using the spherical integral criterion and the radial integral criterion to optimize the Sigma point sampling strategy and the weight distribution of the UKF. Secondly, according to the characteristics of the aircraft leveled off with a constant velocity, the lift and drag coeflcients can be computed and the flight envelope is established to reflect the flight performance. Lastly, the simulation results verify that the proposed method is practical.