To establish more accurate and effective gun dynamics model, which considered fully the coupling influence of projectile and flexible barrel, by introducing the virtual substance into the barrel system, a modeling method for self-propelled guns was presented. Taking a tracked self-propelled gun for an example, combining Ansys and Recurdyn, a multi-body dynamics model was built up. The correlation curves between the experimental data and the simulation result under the same firing conditions were obtained. The results show that the model can describe accurately the dynamic response and the projectile movement, which can provide theoretical references for the further research on gun launch dynamics characteristics.