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 using the Euler-Euler method. The results show that the characteristics of the three-dimensional flow field in the bore are 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. Additionally, the absolute value of the initial negative pressure difference between the breech and the forcing cone decreases from 1.62 MPa to 0.76 MPa. The start-up time of the bullet is extended from 2.82 ms to 2.94 ms, and the time required for the forcing cone pressure to reach its peak is increases from 4.04 ms to 4.20 ms. At the same time, complex three-dimensional pressure fluctuations were observed in the chamber. Before the bullet movement, 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 movement, the chamber pressure consistently decreases along the X-axis direction. However, along the Y-axis direction, the pressure in the chamber remains essentially 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 a constant level, 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.