Abstract: Launching mode，which achieves the same effect as launching in the air by opening several grooves on the inner wall of barrel to guide a part of the combustion gas for real-time drainage， is proposed for exploring the underwater gun fire at low-resistance and its working principle.A three-dimensional unsteady two-phase flow model for the multiple gas jets in the liquid tube is established. The interaction process of gas-liquid in 30mm liquid-filled circular tubes was numerically simulated.The results show that the multiple gas jets converge along the circumferential direction during the expansion process，and then cooperate to discharge the liquid near the wall.Finally，the multiple gas jets converge radially and form a columnar gas curtain. Axial expansion speed of multiple gas jets in the circular tube decays briefly and rapidly first and then changes from fluctuating rise to nonlinear rise.In the near-field of the orifices，the gas area fraction of each section increases gradually with fluctuating. In the far-field of the orifices， the gas area fraction of each section increases rapidly when the gas jets extend to a certain section， and then it rises slowly with fluctuating. The volume fraction of gas in the water-filled circular tube increases non-linearly, and the drainage effect is good.

Key words: underwatergun, underwatergascurtain, wallcombustiongasjet, gas-liquidinteraction, drainageeffect