A reliable dynamic model of air rudder systems is essentially important for the flight control system design. In this paper，the nonlinear modeling of a launch vehicle air rudder system with joint clearance and friction is studied. A four-parameter dynamic model and a six-parameter dynamic model with free-play and friction are presented by adopting Coulomb friction and Stribeck friction， respectively. Their reliability is investigated by applying physical parameter identification method to the frequency response test data. An improved five-parameter nonlinear dynamic model is proposed based on a modified Coulomb friction. It is found that，the parameters of the five-parameter model identified via different tests show better consistency than the four-parameter and six-parameter models. Thus the five-parameter model is more reliable.