The multilevel stepped-wall chamber is designed to study the combustion stability control mechanism of the bulk-loaded liquid propellant gun (BLPG). The cold state experiment of the interaction of the high speed gas jet with liquid medium is conducted by means of high speed digital camera system. The simulated small caliber bulk-loaded liquid propellant combustion propulsion device is designed to study the effect of the stepped-wall chamber size on the combustion stability. The experimental results indicate that, the stepped-wall structure can restrain the expansion randomness of the Taylor cavity and leads smooth expansion at each step. In 4 stepped-wall chamber with ΔD/L = 3/40, the interior ballistic performance of BLPG is stable and the consistency of the p-t curves is good. Two-dimensional unsteady model is developed based on the BLPG combustion propulsion experiment. The numerical simulation results coincide well with the experiment.