The integrated navigation system is used in a land vehicle navigation system. The navigation system is described by state-space model and measurement model. To get precision location information, some parameters of the model is important as well as the appropriate sensors and right measurement method. While some of those parameters, such as process noise can't be confirmed rightly and only can be approximate estimated. In this paper, the process noise is replaced by three parameters and IMMPF (Interacting Multiple Model Particle Filter) is used to resolve this problem. The method is applied in a GPS/DR (Global Position System/Dead Reckoning) land navigation system. The results prove the method has good performances.