For the disadvantage of tracking the soft failure of the residual test based on Kalman filter, this paper makes an improvement on the method by using the state propagator. With the aid of normalizing the residual vector with the normal distribution, each component of the residual vector needs to be tested instead of the whole of that. Thus a higher fault detection sensitivity as well as the capability of the fault identification can be obtained according to the presented method. The simulation shows that the method is effective, by using this method the fault detection sensitivity of the system can be improved to a great extent.