Abstract: In order to explore the influence of the propellant bed accumulation distribution on the three-dimensional characteristics of initial pressure wave in the chamber during the internal ballistic process of a large-caliber modular charge gun, a three-dimensional gas-solid two-phase combustion dynamic model of the modular charge was established. Firstly, solid powder particles were treated as discrete phase. Based on Euler-Lagrange method, the motion law and accumulation distribution of propellant particles under different initial broken sizes of cartridge end caps were simulated. Then, the propellant particles were treated as continuous phase and the evolution of pressure distribution in the chamber after combustion of the powder bed with different accumulation distribution was numerically simulated on the base of Euler-Euler method. The results show that the characteristics of the three-dimensional flow field in the bore is affected by the difference of the initial fracture size of the cartridge end cap. When the initial breaking angle of the cartridge end cap increases from 0° to 120°, the difference of the propellant particles in the area near breech and the area near forcing cone decreases from 12.2% to 0.6% after the dispersion and settlement of the propellant particles, and the absolute value of the initial negative pressure difference between the breech and the forcing cone decreases from 1.62MPa to 0.76MPa. The start-up time of the bullet is extended from 2.82ms to 2.94ms, and the time required for the forcing cone pressure to peak is increased from 4.04ms to 4.20ms. At the same time, there are complex three-dimensional pressure fluctuations in the chamber. Before the bullet moves, the chamber pressure can be divided into four pressure evolution characteristics along the X-axis direction, presenting the pressure with no changing, gradually decreasing, first decreasing and then increasing, as well as gradually increasing. After the bullet moves, the chamber pressure always keeps decreasing along the X-axis direction. However, along the Y-axis direction, the pressure in the chamber is basically unchanged before and after the bullet movement. The pressure in the chamber can be also divided into four pressure evolution characteristics along the Z axis direction, presenting basically maintaining the same, gradually decreasing, first decreasing and then increasing, first decreasing and then increasing and then decreasing. The research results have some reference value for the interior ballistic safety analysis of modular charge guns.