In laser ignition system, to ignite the propellant, laser has to be transmitted into chamber through an optical window.If the optical window is destroyed by high pressure or high temperature, or contaminated by the black powder residue, the ignition system will fail in further firings.The sapphire window can withstand the high pressure and high temperature of the ballistic cycle, but it is necessary to keep the window clean for the laser to transmit repeatedly.In modular charge systems or fully combustible cartridge cases, there is no place to put a shield window like in ammunition with stub base, thus a new contamination prevention method is put forward.In this method, the hydrodynamic force is used to divert the gas flow in a cavity located in the breech to avoid contact with the optical window and prevent it from being contaminated.To verify the effectiveness of this structure, a three-dimensional unsteady discrete phase model (DPM) coupled with an interior ballistic process was established.The simulation results indicated that the flow jet from the direct orifice was diverted, and particle debris did not contaminate the optical window due to the hydrodynamic force.Various factors influencing the contamination prevention effect were discussed, and the structure parameters were optimized.