To study the method of controlling the combustion stability in bulk-loaded liquid propellant gun, a 2-D model of gas jet expansion process in the stepped-wall chamber was developed. The unsteady process of gas jet interaction with liquid was simulated by the commercial software FLUENT. The influences of chamber configuration, jet pressure and nozzle diameter on jet shape were discussed. The isothermal and isobaric graphs and density isograms of gas jet field were obtained. Results show that when the stepped-wall chamber dimension ratio is 0.8, and jet pressure is more than 30 MPa, or nozzle diameter is more than 2 mm, gas jet expansion process is unsteady. Mixing process between gas and liquid is strong. The calculated expansion displacement of gas jet is in good agreement with the experimental data.