Methods for improving the accuracy and guaranteeing the fidelity of numerical predictions are of great importance for physical problems accompanied by strong nonlinear multi-physic interactions and extreme working conditions. The prediction of propellant combustion in a gun launching process is a typical example. A one-dimensional two-fluid model was utilized to govern the fluid field induced by the combustion. In order to guarantee the accuracy of the predictions, we utilized a Roe's scheme to solve the partial differential equations. To guarantee the accuracy of the moving boundaries in the expanding chamber, the coupling between the updating of the fluid field and the approximation of the mechanical interactions between the projectile and the barrel is realized based on the commercial software ABAQUS. The separate treatment of the advection and source terms in the split approach was verified through the agreement between the maximum value of the predicted average pressure in a closed bomb and the theoretical value. The coupled method dealing with the moving boundaries was validated by comparing the numerical results with the analytical results of a pressure-driven piston case. Finally, the numerical method was applied to a large-caliber gun. The predicted maximum pressure and the muzzle velocity agree well with experiments.