Abstract: Aerodynamic coefficient identification for time-varying aircraft system has been studied. Usually, the time-varying system identification is quite difficult. On the basis of practical measurement in flight tests and by analyzing trend of the coefficients in aircraft time-varying differential equations, it is possible to transform the individual time-varying coefficient into a known function multi- plied by an unknown constant. These unknown constants are referred to as undefined coefficients. With the aid of the Newton-Raphson method extended to the time-varying coefficient differential equations the undefined coefficients can be evaluated by iteration calculation. In this way the complicated time-varying aircraft identification can be carried out.In order to verify this calculation the aerodynamic coefficients of an aircraft have been evaluated by the data taken from one of its unsteady flight, and the reliability of aerodynamic coefficients obtained from this identification has been discussed. The calculation results matched very well the test data and made a contribution to the improvement of aircraft fight tests.