Abstract: A new method is proposed in this paper to use microprocessor system to evaluate the flight qualities of a aircraft. The extended Kalman filtering algorithm （EKF） is used to estimate the parameters of the discrete model of longitudinal equivalent systems. Some new improvements are made for EKF algorithm to prevent divergence of estimated parameters and to reduce the estimating errors. The algorithm will converge to a local minimum because of the reduced order approximation. Satisfactory accuracy of parameter estimation and fast convergent rates have been achieved by both digital simulation calculation and microprocessor system experiments using arbitrary input signals The experimental system works stationarily and has good repeatable characteristics. That proves the method we have adapted and proposed is suitable.