Abstract: For trans-atmospheric flight vehicle controlled with RCS（Reaction Control System） like spaceship return capsule,the real control moment of RCS in flight condition is very important in the design of control laws.In this paper,the method to obtain the RCS control moment parameters by the use of system identification technique is proposed,the algorithm and mathematic model are derived based on maximum likelihood criterion and Newton-Raphson iteration.The results from simulated data identification and a certain return capsule flight data identification show that the algorithm developed in this paper is really effective not only in obtaining the aerodynamic moment coefficient parameters but also the RCS control moment parameters.