To understand the firing mechanism of underwater gun, a experimental platform was designed to simulate the interaction between cone-shaped multiple combustion gas jets and liquid.The influences from various injection pressures and nozzle diameters on multiple jets expansion characteristics was studied using a high-speed camera system.Experiment results show that increasing the injection pressure seems to improve expansion speed of jets and enhance flow turbulence and gas-liquid entrainment process.Injection pressure rises from 10.8 MPa to 28.8 MPa, expansion speed of central jet of nozzle A increases 28%, expansion speed of lateral jet increases 40%.The center orifice diameter increases from 2mm to 3mm, the maximum expansion speed of central jet and lateral jet increases 29%, 26%, respectively, and the speed decays faster as a another feature, while the lateral orifice diameter has little influence on expansion speed.